I don't see anything fundamentally wrong with that approach on a temporary basis, just be sure to follow typical safety guidelines about extension cords. Use an appropriately sized, grounded, outdoor extension cord. Consider GFCI protection.
Extension cord safety
Sizing extension cords at about.com
Safety groups generally caution against using extension cords in a permanent capacity, or installing inside ceilings, crawlspaces, etc. I presume the primary underlying concern is about routing and protection of the cable to prevent damage. You may have critter-damage concerns in your application.
If you've got such easy access to the crawlspace, what about running a new circuit, or an extension of an existing one, to create that outlet you desire?