SDI loop through with LTC embedder
It is quite possible to make, but think FPGA with 3 Gig SERDES (so probably a modest Kintex 7 or something similar) with a couple of SDI line IO parts (TI) and maybe a PLL from SI Labs, add some power conversion and a mess of VHDL, job done. 

Maybe a few months work.  

Obviously if you have LTC you can stripe that onto an audio track just using any old audio embedder, but I am guessing that does not really do it for you? 

I would be a little surprised if YellowBrix or maybe Decimator Designs do not have a box that does this. 

Regards, Dan.