I want to be able to do something like this to be able to export only valid, non-expired encryption-capable subkeys:

gpg --export-filter 'drop-subkey=expired -t || revoked -t || usage !~ e'

what's missing is the usage option for the drop-subkey filter type for --export-filter.

This doesn't seem to be implemented yet, though Werner mentioned it over a year ago on gnupg-users as a desirable outcome:

This is just a first step; we also need a properties for the key capability.