Hello Fred,
As you cannot create a Mass Data Run Object (MDRO) for a SAP BO you need to define a Custom BO (with query and action) and built your MDRO on this Custom BO.
Either you have as many shadow instances of your Custom BO which will be found by the query or your ABSL code in the action is smart enough to find the Fixed Assets you want.
Then this action is changing the useful life (or whatever else) of the Fixed Assets.
HTH,
Horst