Unfortunately I do not have a working LvTTL serial kit at the moment or I would test for you.
One thing to note, and this has been an issue even in all of the 17.x builds, regarding the MTD, is that dmesg will have this:
[19209.009796] print_req_error: 2 callbacks suppressed
[19209.009805] print_req_error: I/O error, dev mtdblock0, sector 0
[19209.014647] print_req_error: I/O error, dev mtdblock0, sector 8
[19209.019955] print_req_error: I/O error, dev mtdblock0, sector 16
[19209.026208] print_req_error: I/O error, dev mtdblock0, sector 24
[19209.032270] print_req_error: I/O error, dev mtdblock0, sector 0
[19209.037537] Buffer I/O error on dev mtdblock0, logical block 0, async page read
[19209.048891] print_req_error: I/O error, dev mtdblock0, sector 0
[19209.050474] Buffer I/O error on dev mtdblock0, logical block 0, async page read
[19209.057359] print_req_error: I/O error, dev mtdblock1, sector 0
[19209.064629] print_req_error: I/O error, dev mtdblock1, sector 8
[19209.070513] print_req_error: I/O error, dev mtdblock1, sector 16
[19209.076079] print_req_error: I/O error, dev mtdblock1, sector 24
[19209.082445] Buffer I/O error on dev mtdblock1, logical block 0, async page read
[19209.091112] Buffer I/O error on dev mtdblock1, logical block 0, async page read
It never causes any problems, but yeah, its in dmesg. I only bring this up in case it has any effect on the ubi expansion, but I will read through the r7800 thread and see what has been discussed and discovered.
Edit 3:
The description for TFTP reset button access given for the R7800 is probably more accurate than mine, thus it seems the R7500v2 and R7800 also have that in common.