I'm playing in a campaign that involves an interdimensional hub of sorts, wherein adventuring parties are given missions and teleported across space/dimensions to the location. Upon completion, in order to return, they are given a spell scroll of Sending by which they contact a dispatcher, who then opens a portal back.

The phrasing of the Sending spell specifies:

The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately.

How are we to interpret this?

For example, suppose a character were to use Alter Self to disguise themselves as someone else, use a different name, forge an assignment, and use that to get to a location of their desire. Upon casting Sending to return, would they be recognized as their "true self", or simply as the person the recipient knows them as?

Is there any means by which an identity can be masked as someone else (or otherwise concealed), specifically in regards to Sending?

Note: There is one dispatcher in particular we interact with the majority of the time who we know well, but there are theoretically others we haven't interacted with yet, if that makes any difference.

No, you can't fool the spell, you have to fool the target another way

The spell Sending specifies that recognizing the sender is a part of the spell's function, and nothing in Alter Self would change that, as it is appearance based.

Sending (as you quoted yourself):

The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately

So if the target knows you, not just your appearance, then it will know you sent the Sending. What you look like at the time is irrelevant, the spell functions as it says it does.

Alter Self:

Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also can't appear as a creature of a different size than you, and your basic shape stays the same; if you're bipedal, you can't use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.

Nothing here says anything about disguising you in any other way besides visually. It does say the sound of your voice changes, but Sending makes the target hear the message in their mind, not with their ears, so changing your voice doesn't affect that.

Now, by what you have described, you'd have to have used a dispatcher who does not know you to begin with and to have already disguised yourself before speaking to them, so when the Sending gets back to them, they will know it comes from the same person they sent out. Basically, you have to fool the dispatcher, not the Sending spell.

To be clear, if your name is Bob the Baker and you have an alter-ego as Skinny Larry, and you cast sending to someone who only knows you as Skinny Larry, Sending will not out you as Bob the Baker, they'll know the Sending came from the 'you' that they were introduced to.

No RAW way to conceal identity

Unfortunately, the specific language of Sending is specific. You've quoted and emphasized the important bit. It doesn't say how they know it's you, just that they know it's you and so they simply know.

The language of Sending only says:

hears the message in its mind, recognizes you as the sender if it knows you

There is no reference to recognizing your voice, only that it hears the message in it's mind and recognizes you as the sender if it knows you. That's it. There is no reference to hearing your voice in it's mind. Just that it hears the message and recognizes it's from you if you're known to them.

Creative Options

But a DM could consider creative solutions and allowing them - but it's going to be a DM call as to whether or not they will.

If I was a DM, I'd likely not allow an illusion spell that changes my physical appearance/voice to hide my entity from magic that communicates through magic. What I would consider are spells/affects/abilities that can occlude who you are mentally.

The Ring of Mind Shielding presents an interesting option. This ring makes the wearer:

immune to magic that allows other creatures to read your thoughts, determine whether you are lying, know your alignment, or know your creature type. Creatures can telepathically communicate with you only if you allow it.

This is the kind of thing that, in my mind as DM, would magically hide who you are in a magical mind-mind connection. However, there may be an unexpected result in communication. Not knowing who someone is is different than a completely unknown caller. It's the difference between a Caller ID you don't recognize and a blocked Caller ID.