For your purposes, I'd say the PC basic figures in green packaging you can still find on eBay quite easily are your best bet (although Susan, if you wanted her, will be a real challenge). I'm no GI Joe authority, but I'm guessing there about the same size and would hold your weapons. For LWW there were a lot of figures issued, but most as StBrian already said were static or had very limited mobility.
I looked at a bunch of my various figures from all the movies and most can't hold weapons. Their hands are either closed or have something already sealed in their hands. The largest size LWW figures in the maroon boxes like the Aslan/Edmund/Cyclops or the Peter/Otmin/Black Dwarf sets and the Disney ones StBrian mentioned are a lot bigger and nicer and I think make the best for play figures of them all and they can definately hold things, but they may be a lot larger than the typical GI Joe? They are also pretty pricy now days. There is a very early LWW set of figures that I only have Lucy and Tumnus of that are probably more the right size to a GI Joe but they are ridiculously hard to find (usually only on eBay UK) and they cost a lot!
And just for correction, there are a few Dawn Treader Narnia toys, though I doubt you'd want them for your purposes. They are also very difficult to acquire I have found unless you live in Australia. There is a set of four VDT toys that were released through a fast food chain known as Hungry Jacks. The figures you could get were Aslan, Tavros, Reepicheep, and the Dawn Treader ship. I have managed so far to get 3 of the 4 off the eBay Australia site. They range from $1-4 but international shipping will hit you hard if you're not local to AU. Sorry, no Narnia kids were in the set and the three animals' ability to do GI Joe battling would be pretty sad, given their small size and cheap creation as a happy meal toy.