Hi, the UDP-203, presumably like the BDP-103, has a supply that provides 5V and +/-15V to the analog stage. The 5V supply is HIGH current, because it supplies the video circuitry and all digital circuitry. It is on the order of amperes, not mA. As a result, if not done properly, in a linear power supply, will result in quite a bit of heat in wasted power. These are raw voltages that are further regulated down to 3.3V for much of the digital and control circuits. I believe the analog supplies may be regulated down to +/-12V.
Now this power feeds ALL video and digital circuitry, impacting DAC, HDMI processors, video processors, etc. As such, it SHOULD make a difference, to replace with a LPS.
The unit is fully enclosed, so the power supply (if linear), must have heat-sinks that conduct directly to the enclosure and reject heat right to the enclosure. SMPS - Switch-mode power supplies are very efficient and inexpensive, hence their use in most A/V players such as this. Their downside is the amount of HF switching noise that they inject into the grounds and every other aspect of the player. The SMPS's that Oppo uses are very well filtered, but it seems logical that if a LPS could achieve the same, it would be beneficial.
I will look at feasible options to address this, but no promises. Others have done it and they are available, at least for the BDP-93 and BDP-103.