CraigKC:
As I continue to research it seems to me the magic is in whether both devices are "open drain" or "push-pull". I've confirmed that the STM6503 is open drain so as I understand it, as long as I can confirm that the DTR on the FT232RL is open drain as well then it is as simple as connecting them both to RESET with no problem.If the RT232RL is push-pull (as I suspect it is but can't find in the datasheet) it looks like I'll need some help choosing the right solution.
Doesn't matter what kind of output pin the FT232RL uses, as this DTR signal is series capacitance coupled to the 328p reset pin, it so won't effect the DC state of the 328's reset pin.
Maybe someone can confirm that this understanding is at least correct? I'll research your idea with diodes. I think I get what you're saying and I will post schematics if I need clarification.
Wiring additional open collector or open drain outputs directly to the 328's reset pin (not to the FTDI side of the series DTR capacitor) should be OK from a DC perspective, however one should really test to see that the AC coupled DTR pulse from the FTDI is still functional after the additional devices are wired to the reset line.
Lastly you should be aware of the 'reset lock-up' problem that was discovered sometime back that requires a diode to be added to the reset pin and wired to +5vdc with cathode end wired to +5vdc. You can see this diode has been added in the latest Uno rev3 schematic on the hardware section of the arduino site.
Lefty