Triggering a build off a Perforce label change?

Any ideas on how I can trigger a build when the contents of Perforce
label change?

I'm currently using <p4changelistlabelincrementer> along with
<modificationset>/<p4> for my other normal submission-triggered
builds, but trying to put a label specifier in the view path (of
either, none or both of those elements) does not seem to work.

I'd appreciate any pointers.  Thanks,
-Marc

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