Need advice on TrueCrypt password recovery

[Rolf]

1) Is it possible at all to define custom charsets?

Yes, that's what "-1" - "-4" flags are for.

2) Is it reasonable at all to try to recover this password or will my computer have to run for the next 3 million years? ;-)

Reasonable if you can reduce the keyspace to something attackable within current specs.

3) Does oclHashcat support resuming after a possible machine crash?

Yes, the "--restore" flag is what you need.
Atom/Philipp can clarify how frequently it is written to disk.

4) How do I find out my TrueCrypt volume's hash? I will need this as input, of course.

Say what?

5) Do I need to specify my volume's hash algorithm? How do I find out, if possible?

Of course.
There is no way to find the hashing algorithm out, except remembering it.

6) I will think harder about my possible password so I can optimize my attack and use permutation mode or hybrid mode.

Good and probably only choice you have.