This might be an instance where Seahorse's DBus interface could help you. If a user is part of the web of trust already, the dialog could possibly be skipped completely. Also, the key in question could be automatically acquired using the DiscoverKeys method.
In any case, I agree with the other comments regarding your "signature identifier" field in that the full finger print should be shown. Also, signature identifier is not a commonly used term that I'm familiar with and in any case it's a key identifier and not a signature identifier.
Let us (the seahorse devs) know if there's anything in the DBus interface that's missing that would make your life easier.