Creating a custom plugin might do it all you have to do is register a request for accessing their API here
However considering the scalability of their pricing... as Construct is a downloaded by over 200,000 active users and maybe more.... or less.. is not feasible.... unless you have 10,000 dollars laying around.
But instead of using tinypng's sdk or api and trow 10k away for something that already is out there for free, just not in a simple interface ... and if you want not only to save cash but you understand how plugins work in Construct you can trow an eye at pngquant and try port it into a plugin for Construct that would be basically like tinypng ... however i think easier would be to just squash manually the images using tinypng or pngquant.. then just replace the spritesheet Construct exports and replace it in the zip folder, i mean is just 5 minutes of work.
I find Construct's Brute image compression and Image Repacking very space saving though... Construct already squashes various alien looking things/images/sounds pretty hard.