Extract the texture from source materials.gcf (I think it's in hl2\materials\models\headcrab_classic) and modify it (you'll need VTFEdit and an image editing program).
Put the modified .vtf in steamapps\<user>\half-life 2\hl2\materials\models\headcrab_classic\
I'm pretty sure that the Valve Developer wiki is the only place with any decent information.
I'm not really familiar with the code, but I'd find where the picking up and dropping code is make it pickup only, then duplicate it to work with another command (+drop or something like that) and make...
http://en.wikipedia.org/wiki/Data_Execution_Prevention
There's a guide on how to disable it for specific processes here: http://www.0xc0000005.com/dep-exceptions.html