I'm wondering if it's possible to desolder all smd components on a PCB using a reflow oven? My plan is to hang the PCB upside down over a piece of aluminum foil, use a standard reflow profile and let gravity do the work. Saving the components is not a concern, only desoldering them. What would prevent this plan from working? Will surface friction prevent some of the smaller components (e.g. 0402 or 0603) from dropping?
No, that's not likely to work. Generally, for small parts, the surface tension of the solder is sufficient to hold the parts in place.
In fact, for two-sided boards, that need to go through reflow twice, small parts that will be upside down for the second pass do not need to be glued in place.