For the "modified" synth, the relevant changes are: (1) the removal of the Polysix's 8049 microprocessor that had been the key assigner, (2) the addition of a key scanning microprocessor from Keyparts UK (measured as 55.3 mA from 5V), and (3) the addition of an Arduino Mega to do all the key assigner functions (measured as 58.3 mA from 5V). When testing in the "de-modified" condition, I disconnected the key scanner and the Arduino. I could not, however, put the 8049 back into the synth. Therefore, the "de-modified" condition under-represents the temperature of a stock Polysix.
Looking at the graphs of the temperature versus time shown at the top of this post, you can see that the test of the modified Polysix reached a final temperature around 148F (63C) whereas the test of the de-modified Polysix reach a final temperature of only 136F (58C). Clearly the 570 mW of the key scanner and Arduino have an impact. That's good to know. Also, this data tells us that the final temperature of the synth varies as 17.7 F per Watt. That'll be a useful number in a moment.
Now, to estimate the temperature of a stock Polysix, I can take the temperature from the "de-modified" test and add in the additional heating due to the power draw of the 8049 microprocessor. Based on the 8049 datasheet, it says that it draws 30-70 mA from 5V (ie, 0.15 to 0.35 Watts). With this added power draw, I estimate that the final temperature will be (136F + [0.15 to 0.35 Watt]*(17.7 F/Watt)) = 138 to 142 F. Let's pick the middle value and call it 140F (60C).
OK, now I can finally answer the question as to whether my mods are putting my power supply at risk. My modified Polysix reaches 148F. Compare this to my estimate of the temperature of a stock Polysix, which is about 140F. That's a reasonably big difference in temperature. While maybe this difference is OK, I should probably not add any more components that would further increase the power draw.
For example, I have this idea of adding wavetable oscillators to the Polysix. If I do this using six discrete Femtoduino microcontrollers, this would probably require ~120 mA from 5V, which would be 600 mW. Adding this to my already-modified synth would result in a final temperature around 156 F (69C). Now, if I were to play this synth in a hot room (85F) instead of at cool conditions of my test (75F), that would mean that my power supply would be running at 156F + (85F - 75F) = 166 F (74 C). That's got to be too hot. My conclusion is that, if I want to add these components, it looks like I'll have to add my own power supply.