A (native) signature capturing control is available in RhoMobile, and could just be the thing you are looking for. But the main draw back of this control is that it does not integrate into your carefully styled HTML5 interface. Luckily HTML5 provides the <canvas> element which allows you to make your own signature capturing control (like I did), or use an existing one like jSignature.
My experience is that a (lightweight) HTML5 control also works faster on both iOS and Android tablets (the latter only if you turn on hardware acceleration).
thanks for the tip and the code Gerbrand.