Not allowing this makes stuff like anvil damage, colour, wood type, live/dead bit, wet/dry etc all too much hassle to deal with.
Naturally I want to get rid of this shit altogether, but first it's necessary to construct a new system that we can shift into before all this bullshit can be addressed fully, so for now we have to work within the bounds of the old system.
This change will permit dynamic colours for concrete/concrete powder etc, dynamic wood types where the wood type isn't embedded in the legacy ID, and so forth. Allowing full flexibility requires either more old system hacks or completing the migration to a new system which doesn't have these limitations. I prefer to do the latter, but this change will make it somewhat easier to do.
