I have a state flow where a serial transmit task blocks first on having anything to transmit and then on a semaphore given by a timer interrupt. The timer fires after a required bus activity period. However, if there is a RX character after the semaphore is geven by the timer interrupt, there is no way to take it back from the RX ISR.

The TX could immediately check for serial activity after unblocking, but the RX event could complete before the TX unblocks, still being within the timer interval. I really need the RX ISR to take back the semaphore each time.

Since "give" is aliased to xQueueGenericSendFromISR, I should be able to take it back using xQueueReceiveFromISR, right?

Yes. Ensure to only use the FromISR versions and absolutely never do anything that could cause the ISR to attempt to block.