Hmm, you could probably connect a monostable multivibrator to the TxD line of the sled, and change the RC time to be in the order of a couple of minutes. Enable notify=1 and the data should be enough to retrigger it. Then the kernel would just need to be changed to pump out the occasional data bit during standby (a byte every minute would do it). I'd suggest masking this standby byte with "Apps use serial port" to prevent data being sent to eg GPS devices.