Greetings.
A summary, for the archive and for google....
The missing piece, from my point of view, is that it looks like the group selector, for the olcLimits option (which is what I started off looking at; and see slapd-config(5)) has similar semantics to that for the corresponding olcAccess option, more fully documented in slapd.access(5).
In the documentation of the <who> field there, we learn that 'The statement group=<group> means that access is granted to requests whose DN is listed in the group entry whose DN is given by <group>.' But despite slapo-dynlist saying 'Any time an entry with a specific objectClass is being returned...', this does _not_ apply here, since the next paragraph of slapd.access says 'For dynamic groups the attributeType must be a subtype of the labeledURI attributeType. Only LDAP URIs of the form ldap:///<base>??<scope>?<filter> will be evaluated in a dynamic group, by searching the local server only.' That is, the olcAccess group processing is, in effect, restricted to the three-argument version of the attrset option of slapo-dynlist -- that's what I had missed.
Presuming the olcLimits option has the same restriction, then the effect I was initially aiming to achieve -- setting a limit for members of a particular group which is dynamically populated -- is not possible for me by this route.
The groups I'm aiming to set limits and access for are most naturally defined from the union of other groups. Such groups are easy to define via the two-argument dynlist-attrset value (which uses ldap:///<base>?member?sub?<filter>), but not, as far as I can see, via the three-argument one. I can probably instead synthesise the groups I want, dynamically, by introducing a memberOf attribute attached to the groups' members, but I worry that has the potential to get a little messy in practice; I notice group.expand, which might help.
I notice that the documentation of olcAccess doesn't actually mention the dynlist overlay, and thus may be entirely independent of it. Something for me to investigate.
Best wishes,
Norman