I came to file a task to request this. While we may lose customisation for a brief while, it is not that hard to fix and to the contrary it can be benefitial to give specific instructions to the blocked users depending on whether they're personally blocked (account), or it's their IP or IP range, by further customising them if needed. Unregistered contributors can be given specific instructions on how to appeal their IP (range)blocks using the appropriate workflows, and the same goes for registered contributors where usually the path does change a bit depending on the wiki.

It looks like since this Task was filed, further block messages have been split from blockedtext e.g., blockedtext-composite-.* too. See current "blocked" messages.

Thanks.

This is already possible and being done on enwiki by testing the $7 parameter for whether it's an IP address. See https://en.wikipedia.org/wiki/MediaWiki:Blockedtext.