FM transmitter power level is locked in kernel, what one need is modified kernel module. What PR 1.3 fmtx kernel module does is to expose new sysfs property 'lock'. fmtxd sets power level of 112 (at least for my region) and afterwards writes '1' to lock, which disables any further writes to power_level sysfs property until reboot. I've modified fmtx kernel module and right now using it. Good news is that tx power level no more changes when charger is connected to phone, thus making possible to set tx power only once during boot. later will post modified module plus startup script for setting power level.