postprocess the libvirt domain.xml ?

[teto]

Some of my (dev) VMs crash from times to times (business as usual). I would like to generate from nixops a domain with settings as described in https://gymnasmata.wordpress.com/2010/12/02/setting-up-gdb-to-work-with-qemu-kvm-via-libvirt/ (e.g. add in the top-level tag the xml rules, and add the qemu command line tags), which is impossible to do without editing the domain xml via virsh.
It's not realistic to support all libvirt options but maybe we could have a postprocess command ? (but it becomes impure I guess). I suppose most people would just use the plain Qemu command but it can be tough to write it at some point and I like the nixops deploy. Is there any alternative to just maintaining my fork ?

[teto]

Or the other way around, would it be ok to have a setting to load an alternate base XML ?