This tool shows the frequency and number of samples for a sample loop, and allows you to set the loop by specifying the frequency or number of samples. The “Move” option specifies which loop marker to move when setting a loop. Press “p” to show the preferences pane.
(idea from Jonas, here.)
Additional features in the works…
Thanks mate!
Got an error when making the smallest loop possible:
Oops, it worked for 1 sample loops, but not for smaller ones. Fixified, and uploaded new version in top post.
Thanks for the quick fix, love that you can set the frequency in the little gui as well 
Are you planning to implement notes/octaves/cents as well, so you can say A and specify the octave and the tool auto lengths the loop to the corresponding frequency?
Yes, as well as a “transpose” feature. Right now, the frequency info is only valid for the basenote of the sample, but with a transpose feature you could, say, make the sound loop at 480 Hz when played 7 semitones above the basenote, or, similarly, make it loop at a G#-5 +43 cents (once the note stuff is implemented) when played 3 semitones below the basenote. You get the idea.
Smart stuff! Looking forward to trying that out.