Replying to my own post here, but for anybody looking for alternative power cables, a Google engineer along with a volunteer have put together a great doc rating many of the options out there. Remember, for a 2016 15" MBP, you want cables rated up to 5A/100W:Not quite a dongle, but does anybody know if there are any USB-C cables other than Apple's that can fully power the 15" rMBP from the 87W brick that comes with it?
https://docs.google.com/spreadsheets/d/1vnpEXfo2HCGADdd9G2x9dMDWqENiY2kgBJUu29f_TX8/pubhtml#