A very long shot - but it might be interesting to see how the R-Brick PROM and L1 controller exchange system serial number <SSN> information. The question will be how the firmware and PROM check the R-Brick's local system serial number.
While working out how the L1 and PROM identify the difference between an O350 and a Tezro rack system, bri3d and I discovered we could change the L1 system serial number prefix from the "M" the O350 uses to a "P" required by the Tezro rackmounted workstation <using an the L2 controller> to "change" the O350 into a Tezro. viewtopic.php?f=3&t=16721235
Admittedly the O350 didn't have serial security enforced, but might that mean it's possible to change the letter prefix of the local system serial number on the R-Brick without affecting the serial security check? Since the R-Brick and the O3x NUMAlink Module are so similar might the serial number prefix be the only difference <like switching the O350 to a Tezro by changing the serial number prefix>?
If you can change the serial number prefix without triggering the R-Brick's serial security, you could use the L2 controller to assign that "M" prefixed serial number to each of your O300s so they'd work with the B-Brick.
To find out you'd have to be willing to risk bricking some hardware
Start the L2 controller with nothing attached.
Record the L2's current serial info <in case you'd like to restore it later>.
Change the SSN in the L2 controller to the R-Brick's local system serial number - but change the "L" prefix to "M" <e.g. "serial set M1001241" - using your R-Brick's local serial number from an earlier post>
Connect the R-Brick and see if will accept the M prefixed serial number.
If the R-Brick accepts the M prefix, leave that serial number in the L2 controller and use it to assign the number to each of your O300s.
If the R-Brick security won't allow a change to the letter prefix, an equally long shot might be to change on the the O300s to the "M" prefixed R-Brick serial number <while it's still in the L2> and try that as well <though I'd try it by starting everything from a cold boot>.
Another approach might be to try a different L1 controller in the R-Brick. It would appear from a google search for L1 part numbers that basic L1 hardware is shared across platforms - all of the ones I found were 030-1439-00x. Get one without the serial security set - say from a C-Brick. Install that into something besides the R-Brick and use the L2 controller to assign it the same serial number the R-Brick uses, but with an O300 friendly "M" prefix. Then use the L1 command "brick type r" to set the correct brick type <do this *last* - just in case a brick type of "r" auto-enables security>. Remove the newly configured L1 and install it into the R-Brick - if the L1 firmware is new enough to sense the presence of the R-brick and auto-enable serial security, hopefully it will lock in the now O300 friendly "M" serial.