If you are able to boot openwrt then your bootloader works and there is not need to reflash it. If you cannot boot any firmware then you can only reflash the bootloader through JTAG.
Can I use openwrt to access my router's boot loader, so I can reflash it? (through the wan port)??
Thx in advance
You can reflush bootloader only through JTAG cable. Why are you interested to flush out the bootloader?
This rather sounds like the builds you used were broken.
If there is anything with the bootloader then you cannot boot any firmware.
To break the bootloader you would have to manually overwrite it, a web firmware upgrade never writes to the bootloader partition and cannot break it.