I think at least one of the F-Droid contributors could find some time to contribute to the Debian images if we have direction on what kind of solution you'd accept here. Sounds like you've provided some ideas already.
Looks like VMDK is still not resizable, but it is possible to convert to VDI then back. We're aiming to move entirely to libvirt anyway, so a fix only in libvirt will still be very helpful.