I know the reasons why dealing KG & PP is unlikely, but if there were a scenario, how about to OKC? Playing supporting roles on a title contending team might appeal to both KG & PP and the OKC management. And the Thunder have assets the Celts would want. This assumes OKC doesn't want to resign Kevin Martin (currently at $12 M) and would require the Celts to take on Perkins $9M contract to make it work for OKC.
The Thunder have 2 first round picks, one which will be in the 10-12 range from Harden trade and their own late in the round. Talented rookies Perry Jones (more likely) and Jeremy Lamb (less likely) could also be included in some combination with picks and Perkins.
This move would lower the Celts cap number and maybe allow signing a quality free agent in a sign and trade that included less essential players like Lee, Bass or Terry.