What about reworking the joints with a stikfas style ball joint? (Similar to Gregorbian's work on GG bodyguard.) You'd probably need to butcher at least 2 IG-88's to get it right (assuming you're not going to try casting).
Has anyone ever seen a super articulated IG-88 droid made? Just curious since the only ones I've ever seen are hasbro's neutral posed versions. It would be really sweet to see him in a firing pose!