I looked at the example script that they have for Evernote, and combined it with our NetNewsWire-to-BookMacster script.  The idea is pretty simple.  Write a script which gets a title, URL and some additional text from PopClip, and creates a new bookmark in BookMacster with these three values as the bookmarks's Name, URL, and Comments, respectively.  Something like this should work…

set aName to "{popclip browser title}"
set aUrl to "{popclip browser url}"
set aComment to "{popclip text}"

tell application "BookMacster"
     land new bookmark name aName url aUrl comments aComment
end tell

Apparently, this script is run in some kind of environment which assigns the name, URL and some other text to those "{popclip whatever}" thingeys.  I don't know how those work.  I blindly copied the doublequotes and curly brackets from PopClip's EverNote script.  My script assigns these to the variables aName, aURL, and aComment, then, finally assigns them to a newly-landed bookmark in BookMacster.

Writing the code as I have done above is probably easier than what I am leaving to you, which is to figure where to put that code and how to "install" it into PopClip    Then, there's testing and fixing.

Thanks for the suggestion; I'll investigate

I'm really enjoying an OS X extension called PopClip. In its basic form it pops up an iOS-style menu of options when you select text. Typically you get Cut, Copy, Search, and Define options, though if the selected text is a URL you can go there with a click.

Here's the beauty part: they have a big library of extensions to add other functions to the pop-up menu. See this: http://pilotmoon.com/popclip/extensions/

And I thought how nice it would be to have an extension to copy a URL into BookMacster.

I've already suggested this to the folks at PilotMoon, but just wanted to mention it here, too.