It might work, providing you ensure that you never output a logic 0 for 11 or more consecutive bit periods.

Maybe it is a better idea to use the TxD pin instead for this, and temporarily disable the UART during your sound generation routine?

Of course, both methods are very risky in that you are messing with either the UART or the RxD pin for a certain time period. What happens if your software crashes during that time? You might not be able to get back into ISP mode.

Have you taken a look at the pulse entry method of entering ISP mode?