Thank you for your really good answers and advise mikegi!

As the wallet containing the z-address I mined to does not contain coins, I don't have to transfer anything before I can send the .dat file.
I'll go ahead and follow those steps:
- make a new wallet on a VM and generate a t-address
- send the old wallet and the new t-address to eXtremal
- backup the old wallet just in case, even if it's very unlikely any coins end up in it

Dikyx