I thought it looked promising that Dawnguard adds a separate Perk Tree for vampires and werewolves, but only when in that form. It made me wonder, would it be possible to do something similar in a mod?

Would I be able to access/bring up the custom tree from a scripted menu instead of disabling normal Tab button functionality?

If not, is there any plan to enable a system for adding custom perk trees?

I really like the idea that performing specific actions level your custom perk trees. Like with vampire lord, using your abilities to "feed" on NPC's gets you perk point progress.

It would be a huge boon to modders if Bethesda enabled some interface for tracking specific actions to advance a custom perk tree. For example, having a certain keyword send a value to the custom tree through a script, when a certain action takes place, so that the value is added to progress in that custom perk tree.

Please please pelase, Bethesda!

Well, I checked around in the CK and can't find anything in the Dawnguard expansion that would allow this. It must be handled in scripts. I suppose we won't know if it is something we can use until we can take a look at what those scripts are doing. Hopefully they'll release them soon.