BTW, are there any properties available to filters outside the ones of route's they're processing?
There are global constants. There will be some more properties of the protocols involved. Global variables won't be supported as it would break parallel execution of filters. If you need something really special, write your own protocol. (This should be also simpler, we should clean and better document the protocol API.)
Are there any some "global variables"?
Can users introduce some?
From 2.0.3 on, it is possible to have custom route attributes, anyway, I suppose this is something you aren't looking for.
I also have a suggestion in regards of "check link": if it would be not only a switch, but, say, reference to an arbitrary interface would be supported, it would also extend Bird's flexibility significantly.
Might be a good candidate to couple with that "interface properties" feature. ;)
Let's see what is feasible after the roa checks are fixed. Feel free to remind this topic then. Maria