The Chicago Manual of Style, 14th Edition contains the following (on the hyphenation or otherwise of compounds):
6.38: The trend in spelling compound words has been away from the use of hyphens; there seems to be a tendency to spell compounds solid . . .
One could argue that the word 'solid' is being used as a predicative adjective here (as in the lake froze solid) - but the semantic analysis is indeterminate, in my view, between adjectival and adverbial usage of 'solid' in the CMoS usage.
AHD lists the adverb polyseme:
solid adv. 1. As a whole; unanimously: The committee voted solid for the challenger.
2.. Without a break or opening; completely or continuously: The theater was booked solid for a month.
To my surprise, Collins doesn't list it, though 'booked solid' is very common in the UK.
What word-class would people plump for in the CMoS usage - and, more importantly, what verbs (other than obvious link-verbs) may 'solid', when not obviously accompanying a noun, follow?