Although there are some differences between GoRC and RC, I hesitate to modify the syntax here. If this were attempted, it also looks like it may be better to deal more directly with something like
nameID USERTYPE type ...
with USERTYPE as a type identifier with the type name following on the same line, instead of a #USERTYPE directive elsewhere earlier in the file (RC would skip over USERTYPE with blank #define USERTYPE). I do foresee some complexities, but still doable.
You currently have the /nw command line option which supresses warning messages. This however does so for all warnings.
As an alternative, it would be quite easy to have a /nwu command line option which supresses the warning message specifically for a user-defined type (however, a typo such as ICAN would be treated as a user-defined type without warning). Would this alternative seem reasonable enough?