@shelly-goelThank you for the links. After a bit more searching of the documentation: to solve this, when using getOffers I had to set the consumerId key to a unique value (it was defaulting to 'undefined' even if I was request regional mboxes). That value is used to determine whether the API should generate a new SDID or if it reuses the SDID already available on the page.

Thank you @shelly-goel Just tried with a local mbox and a new sdid is being generated. Would also rather avoid using logging:client_side It seems the sdid is decided by a function called getSupplementalDataID - and there are two in circulation at any one time (stored in visitor._supplementalDataIDCurrent or visitor._supplementalDataIDLast). If I override those values I can force my getOffers request to use the sdid of my choosing (the same one that was used for pageLoad and Adobe Analytics page ...