Many thanks to all who responded, most particularly to Richard Sims, who
(no surprise) steered me in the right direction.

In brief, on a whim, I restarted the TSM server (potentially costly whim);
this didn't help, but it did change the error
tsm: ADSM2>def logcopy /dev/rD100b2L01L002 /dev/rD100b2L01L002
ANR2442E DEFINE LOGCOPY: Volume /dev/rD100b2L01L002 is not a defined
recovery log volume.
ANS8001I Return code 11.

tsm: ADSM2>def logvol /dev/rD100b2L01L002
ANR2452E DEFINE LOGVOLUME: Maximum recovery log capacity exceeded.
ANS8001I Return code 14.


tsm: ADSM2>vary on /dev/rD100b2L01L002
ANR2422E VARY ONLINE: Volume /dev/rD100b2L01L002 is not a defined disk volume.
ANS8001I Return code 11.

The only documentation for ANR2442E I could find on-line was for the DELETE
LOGVOLUME command - no help there.

Richard, however, had mentioned the /tmp/adsm.disk.dev.<raw name> file

So I looked:
$ ls -ltr adsm*
-rw-r--r--   1 adsm2    adsm2            19 Jun 28 13:48
adsm.disk.dev.rD100b2L01L001
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD200b2L01L002
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD200b2L01L001
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD101b2D01D004
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD101b2D01D003
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD101b2D01D002
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD101b2D01D001
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD111b2D01D006
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD111b2D01D005
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD111b2D01D004
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD111b2D01D003
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD111b2D01D002
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD111b2D01D001
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD101b2D01D006
-rw-r--r--   1 adsm2    adsm2            19 Jun 29 13:32
adsm.disk.dev.rD101b2D01D005

The later timestamps corresponded to the recent restart.

Noting that there wasn't one for rD100b2L01L002, I decided to try again -
and this time...
tsm: ADSM2>def logcopy /dev/rD200b2L01L002 /dev/rD100b2L01L002
ANR2262I Recovery log volume copy /dev/rD100b2L01L002 defined;
synchronization process started (process ID 3).
ANS8003I Process number 3 started.

Huh?

So I renamed the adsm.disk.dev.rD100b2L01L001 file, and
tsm: ADSM2>def logcopy /dev/rD200b2L01L001 /dev/rD100b2L01L001
ANR2262I Recovery log volume copy /dev/rD100b2L01L001 defined;
synchronization process started (process ID 4).
ANS8003I Process number 4 started.

And now the world is as it should be:
tsm: ADSM2>q logvol

Volume Name          Copy       Volume Name          Copy       Volume
Name          Copy
(Copy 1)             Status     (Copy 2)             Status     (Copy
3)             Status
----------------     ------     ----------------     ------
----------------     ------
/dev/rD100b2L01-     Sync'd     /dev/rD200b2L01-     Sync'd
         Undef-
 L002                            L002
           ined
/dev/rD100b2L01-     Sync'd     /dev/rD200b2L01-     Sync'd
         Undef-
 L001                            L001
           ined

On to the next issue..