I checked out your add-in. It seems you've done pretty good work there. Keep it up that way!
We are definitely planning on a plug-in API. It's not top priority task right now, but we'll start implementing it soon enough since our user's community shows quite some interest in it (http://justdecompile.uservoice.com/forums/113277-justdecompile-feature-suggestions/filters/top). That being the case, we cannot really provide even a preliminary version to you in this moment. We're still in a planing stage and there is really nothing you could make use of.
We'll announce the API once we have it ready, so, please stay tuned.
the Telerik team
P.S. The chances are, that by the time we release the API we'll have built-in features that do a lot of the stuff your plugin does now. We already have the very same search you provide (ours actually searches for a given type in all the assemblies loaded inthe list as opposed to just searchin in a single assembly) and we have planned on navigation throught the inheritance hierarchy similar to the one Telerik JustCode has. So, you might wanna check out what functionality we have at the time we release the API before you invest time in porting your plugin for JustDecompile.
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