ok here is my opinion
first of all asking to film her feet is almost out of the question. you can go about it if your sneaky enough however.a lot will depend on how much of a bond you two have. next if your on friendly terms with her offer her a foot massage preferably with a hidden camera rolling. if you get cought however you may need a lawyer or a first class medic, perhaps both.
most gals will not pass up a foot massage from someone they trust.but the magic word is trust. if she trusts you and finds out you filmed it without her knowing you can kiss any trust or good will on her part good bye.
if you would like to make this work you might want to talk to her about it and be honest. perhaps she has a few quirks of her own. maybe you might swap fantasy's or pet perversions.
best of luck but keep in mind if you get to the massage and slip in a tickle bee prepared to duck.. and perhaps pick a few teeth off the floor, never let a tickled foot get to close to your face