Two parts to this idea:
I know you can type RPE to the notes field but how about formalising the data with a 1-10 drop down box, like the option on rides where you have not used a power meter.
On the drop down 1-10 mentioned above, have the optional tick box to use the subsequent estimated TSS instead of that calculated from your power meter. This would be useful where a power meter has not been calibrated correctly, suffered drop outs or run out of battery. I have a small number of rides in my calendar where the TSS is wildly inaccurate because of this.