So it seems likely a Bitcoin ASIC can't grind nip-13 proof-of-work. Or can it, with AsicBoost like magic?

The event id is manipulated to get leading zeros (in binary). This is supposed to be done by adding a nonce tag and then rotating that, but a miner could of course hold the nonce fixed and grind something else.

Assuming you must feed it 80 bytes, the equivalent of a Bitcoin header, the last two characters have to be "] which means only 1 in 256 nonces (the last 4 bytes of the header) produce a valid nostr message. Or can the chips be told not to grind the last two bytes?

But most importantly you have to be able to feed the asic the first part of the message as a midstate. If you can do that, then I think the message can be of arbitrary length. But I don't understand well enough what these chips can and can't do. nostr:note1557d4thw7flutjhs8gl9ql4m66m64xw6xpre4lt06ca57h53df0qf7735a