...then when you have your track marked that has a booklet assigned you "send" it to your external tool
This works well in Standard View. It's also possible (and more convenient) to define a link that will open [booklet] in an external reader. Using that technique, [booklet] could also be used to save a URL to a similar Web reference. In that case, separate links would be defined to handle PDF vs. HTTP references.
Unfortunately, there's no equivalent to
Send To or
Links in Theatre View. The internal browser (in Theatre View) can display a PDF if it has a PDF reader plugin. If that's the case, a "Webpage" item will display a specific PDF, but that doesn't help display those referenced in [booklet]. What's needed is a way to define "Links" for a view. The "Search URL" (to use the same term as the existing
Links feature) could be a Web (http://) or File (file://) reference, and take any parameter associated with the selected file (like [booklet]). As such, it would function as a
Link or a
Send to item. There would be a "Links" item on the main roller, with a sub-menu listing any applicable links.
It is possible to import PDF's from audio folders and define a view that will include them. But it seems to me that will mess-up audio playback. Putting them in a separate booklets view would prevent that, but defeat the purpose. The reference needs to be available in the context of the album and any of the tracks it contains.