Ok, I tried it on mine. I have rewritten the whole partition with nokia_osbl.mbn extracted from RM803_12w07_prod_generic_updated_nokia_osbl.esco. It booted just fine and Nokia Care Suite flashing works again.
Here are the exact steps I used to recover the Nokia DLOAD bootloader after unlock:
1) Download _http_://nds2.fds-fire.nokia.com/p/d/fds_fire/1203/2211/7000248136/RM803_12w07_prod_generic_updated_nokia_osbl.esco
2) Rename the file to RM803_12w07_prod_generic_updated_nokia_osbl.zip and open it. It should contain two files, one will be named nokia_osbl.mbn. Extract that one.
3) Connect the Nokia 710 phone in the Qualcomm bootloader mode to the computer (ie. boot it with holding the volume up key) and wait till it shows as a disk.
4) Open HxD as administrator and select Extras -> Open disk. Find the Removable disk that matches the connected Nokia device. Don't forget to uncheck the "read-only" mode! The partition with the boot loader should start at sector 1001, you can navigate to it using the toolbar at the top.
5) Verify that it starts with the bytes 0B 00.
6) Open nokia_osbl.mdn in HxD and copy the whole file over to the disk starting at the sector 1001. Save it.
7) Unplug the device and reboot it. Now it should be flashable with NCS again.