If I purchase an ASIC (hypothetically) and then for whatever reason Bitcoin disappears, becomes defunct or otherwise, is there anything else I could possibly use it for, such as SHA256 cracking - or is there no other possible use and does it become an elaborate doorstop?
No, Bitcoin ASICs can not be reused for any other purpose, unless something were created that uses the exact same hashing technique.
Unlike what some ill-informed radio talk show hosts say, the mining network is cryptographically incompatible with SHA256 password cracking, the block chain itself is not a conspiracy by the Russians as a rainbow table.
Besides most hashed passwords use a salt, and the manner it is typically done is incompatible with what is being done in Bitcoin.
So, no. The ASIC will forever be exclusive to the Bitcoin unless another RFC or standard requires a proof of work (a la Hash Cash) for some other purpose.