think the dr scientist tremolessence has those features:
http://proguitarshop.com/dr-scientist-t ... sence.html
two expression inputs for either controlling speed or depth , looks like the ticket , can also set the trem for momentary or latching, for glitch like effects , very cool
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otherwise there's small ones with a tap tempo, this one perked my interest definitely:
http://www.copilotfx.com/polypus.html
or this has an expression input for tremolo and a tap, but much too large i think for what you're after
http://www.pedalspluseffectswarehouse.c ... ls-tr1.htm