Well, all that I can say is that it is not smart to put an emu ID into RAM at all Why? Simply, because it can be overwritten by any program any time. I hope I don't have to explain further the disadvantages of this
Further, the flags you mention are only standard for the built-in KERNAL ROM, and there's no guarantee whatsoever that other programs will care about that. For example: 1551 presence is simply determined by sending an ATN signal to the two IEC ports at $FEF0 and $FEC0, if there's response, than the ROM writes a flag to the RAM area to remind itself next time that it is available. This check occurs like after every reset signals for example. It's similar to the behaviour of address $C6, which stores the actually pressed key's code, but only for the KERNAL ROM. For other programs that might disable the ROMs (MANY, actually), this is useless (what more, they might want to use that space as free RAM) and you have to use the TED's keyboard latch at $FD30-$FD3F and $FF08 low-level...
Instead, it's more useful to patch the ROM. Namely, in case of the plus/4, there's a small empty ROM area between $FF50 and $FF80 for example. An emulator ID string could be simply written to there by user interaction, while the original content ($FF) could be restored by another click of a button
BTW: cheers for the 1581 drive specs. I am eager to get hold of a D81 file, too
Why? Simply, because it can be overwritten by any program any time. I hope I don't have to explain further the disadvantages of this 