How can/does a mobile phone provide power to pen drive?
That 100ma rating is a maximum, not the normal rating. The pen drive is mostly idle, and should only need to peak occasionally. USB power can support at least 150ma, generally more.

http://en.wikipedia.org/wiki/USB#Power

Connecting a device that draws more power is most likely to flatten batteries sooner, but if it is within the USB power specifications, the phone should support it.

That said - I am not familiar with the specific limitations of OTG.