Since the proposed abbreviation represents a word, rather than a letter like the thorn < Þ, þ > or the eth < Ð, ð >, a better analog would be the symbol for that in medieval texts:
It’s a < t >, hence the cross-stroke, superimposed upon a thorn. It might also appear in manuscripts as the < t > written over the thorn:
Scribes would also write an < e > over a thorn for the or an < s > over a thorn for this, but these abbreviations are less common.
There are hundreds of such abbreviations, so many that there are paleography books that are nothing but glossaries of the abbreviation practices. (Which is one reason that reading medieval manuscripts is so hard; they’re chock-a-block with abbreviations.) But most of these abbreviations went out of use with the advent of the printing press. They disappeared for a reason—if you’re not writing by hand, they’re more trouble than they’re worth. It seems pointless to try an introduce them now. They don’t save much space, either on the page or in bytes, and they’re unwieldy to deploy. Not to mention that convincing people to use them is a hopeless task.
The history of the @ symbol is instructive. For years, it existed as a specialized symbol used in accounting, very useful in a very narrow and specialized application because it signaled the number that followed represented a unit price. Then the designers of ARPA’s email system used it as a component in email addresses because it was already on standard keyboards and bytes were then really expensive, so saving a few really mattered. Since email has become ubiquitous, the @ symbol hasn’t really infiltrated into the language at large. It’s mainly restricted to jocular or idiosyncratic uses. There just isn’t a need for it. “At” is easy to type.