If you create a PCB that can do HDMI officially and sell then you risk getting slapped with a 10 000 USD/year fine + 0,04 USD/port. So a solution that can get around that will do. Also keep in mind that the bit timings on these serial links are really
tightly packed with PLL locked clocks in parallel. This makes it only possible for FPGAs that have really fast I/O at Gbit/s speed and PLL to go with it. Which limits choices severely.
The
chip package 64-LQFP for the 3,3V LVTTL to DVI chip seems handleable. You could try to replace the existing chip with another chip soldered on an adapter PCB. Just watch for wire inductance between the board and the impedance matching, etc.. And don't forget those decoupling capacitances.
A feature solution could be to put the while video converter (LVTTL-DVI) on a little expansion board of it's own. That way it could be inofficially upgraded without anyone being able to claim any responsibility for the base-board PCB creator.