In order to compute the Tendril update, the script loops over for section in mysql_legacy.CORE_SECTIONS, which doesn't include pc1, pc2, or pc3 -- but I don't think the right fix is to add them there. That would also mean we operate on them e.g. while setting the DBs read-only, and then read-write again. With nothing but optimism in my heart, I'm assigning this task over to the Touched It Last owner of the mysql_legacy module, to figure out what needs to happen here. (A reasonable answer is "we'll cover this with the new mysql module" -- this task is just to make sure the cookbook does the right thing, one way or the other.)
Note also that the Cumin query used by mysql_legacy in get_core_dbs() to identify the DB masters won't work without modification, since it starts with "A:db-core and..." and that alias doesn't include the pcNNNN hosts. Very reasonably, since parsercache isn't part of the core DBs.
And one last thing, in passing:
rzl@cumin1001:~$ sudo cumin 'A:eqiad and A:db-section-pc1 and A:db-role-master' 2 hosts will be targeted: pc[1007,1010].eqiad.wmnet
That seems like it ought to only be pc1007, right? Same in codfw.