Re: PIC18F ECAN: Tx buffer registers locked during transmit?
Found it. In the discussion of TXBnCON (section "Register 27-5" of the spec sheet), footnote 2 says "While TXREQ is set, Transmit Buffer registers remain read-only." Section 27.5.4 also says "As with all transmit buffers, once the TXREQ bit is set, buffer registers become read-only and any writes to them will be ignored". No mention of other side effects, so presumably it's literally just that: The registers associated with the transmit buffer in question become read only.
Posting this answer in case someone else searches on this same question.