The FMC knows the weight you have entered, the climatic conditions you have entered and it knows the aircraft performance envelope, hence it can predict its max Alt. However, the 'VNAV engaged' envelope is slightly more restrictive than the aircraft envelope.
If you ignored the message 'Unable CRZ ALT' I suspect it would climb to the top of the VNAV engaged envelope, then drop into ALT HOLD, although like others here I've never tried it, and it's not in the manual.