Skinny legs aren't an Escher-exclusive issue - many Infinity and Reaper figs have the same issue, which means plenty of people have found a way to make it work.
As mentioned, thin pins, equally thin drill bits, and a bit of care render the task perfectly possible, if somewhat fiddly. I'd recommend a wire-gauge drill bit - something sized in the upper 70s - and a head pin (a small size, like you'd use to secure cloth for sewing). They're cheap, quite stiff, and rather skinny. To resist torque and therefore actually increase strength, you really need a bit of depth. With a tight fit, a few millimeters will suffice. Even this usually means going up into the ankle/calf, though, if you aren't talking about chunky Orks and Space Marines. Unless the pose is
really dynamic, you can usually manage this by starting your hole close to the back of the heel and angling it slightly to follow the lean of the leg.
Flattening the sole to ensure maximum contact when it's time to glue goes a long way towards a strong join, too, as does extending the thickness of the base with a layer or two of styrene sheet or a blob of putty on the underside.