Well, I finally got my blinkenlights working to the tune of the teletype machine. Oscar Vermeulen pointed me to the correct value to change in the simh code. The SERIAL_OUT_WAIT value in simh_defs.h needed to be increased from the default of 100 to 20000. Now the accumulator LEDs seem to blink correctly.
UPDATE: After doing some research on SERIAL_OUT_WAIT, I found that it is also used in the lp, pt, and ttx drivers. That means that, for instance, punching a simulated paper tape will also be at 110 baud. Clearly, that is not what we want. So, I changed the SERIAL_OUT_WAIT back to 100 and added a new constant – TTO_WAIT – to pdp8_defs.h, and set it to 20000. Then in pdp8_tt.c, changed the SERIAL_OUT_WAIT in the tto_unit structure to TTO_WAIT. Now, only the console runs at 110 baud.