Currently, there is an exception in the generic bitstream description about the JPEG2000-RCT, because we don't want to break files in the wild for version 0 to 3:
"Exception for the JPEG2000-RCT conversion: if bits_per_raw_sample is between 9 and 15 inclusive, the following formulae for reversible conversions between YCbCr and RGB MUST be used instead of the ones above"

As all files in the wild with this exception are without alpha plane, and because the standard JPEG2000-RCT is known to be better for generic cases (I also tested a bit the compression with some 10/12-bit files I have, I see better compression ~3% without the exception for the few real world files I have, with some exceptions e.g. some artificially created files I have made show a slight worse compression) and also the 16-bit RGB files don't have this exception, I suggest to explicitly stipulate that the exception is for only the files in the wild, by adding " and alpha_plane is 0".

Then it is easy to adapt FFmpeg ("if (sizeof(TYPE) == 4)" becomes "if (sizeof(TYPE) == 4 || transparency)") and other encoders/decoders, and we can demonstrate how versatile FFV1 is.

As having no explicit information is not good for a spec, the other option would be to explicitly say that exception is for with and without alpha_plane, but I don't see a reason to do so (I see usage of different R/G/B coefficients in FFmpeg code for v4 experimental, but for v1-3 we need to choose 1 method for all streams).

Summary:
RGB with bit depth 1-8 & 16+, and RGBA any bit depth: no exception
RGB with bit depth 9-15: exception