Would it be plausible to launch two or more cubsat's into orbit, and then have them dock together, allowing them to pack on more hardware and even fuel, broadening their capability? If it is possible, how much of the cubesat's mass would be dedicated to the docking procedure, and would too high a mass fraction be prohibitive?
There is the joint project of Caltech, University of Surrey and some other companies for development of Autonomously Assembly of a Reconfigurable Space Telescope (AAReST). This project suggests (at the preliminary stage) using a set of about 3U (it much more like 4U) cubesats for orbital mounting of a telescope mirror. The authors use term "docking", albeit this is much more seems to "mounting". There isn't any rendezvous operations (from large distance), but exists a kind of targeting.
The authors are going to use electro-magnetic principles for cubesats docking. A distance of docking estimated as 30-50 cm (from the 2017-year document).
Description of docking mechanism is in AAReST_Payload_CDR_Surrey_Update.pdf (2015) (pages from 39 to 63). Some pictures from this PDF:
And one more from AE105_Final_Presentation_06-05-2018.pdf (2018):