"The parcels all remain in the landlord's renters land group"
Using a different rental group for each parcel you rent out can be a workaround to force max prim usage. But that depends on the amount of parcels you want to rent out, as there is a limit on the amount of groups you are allowed to have (still 25?).
Be aware that using a prim usage warning system seem to encourage a few renters to try out your management skills, limits and patience
The amount of prims come with the size of land and/or the bonus option set in the region tool.
If I understand your question right Raven, you only want one parcel to have more prims then others in the same sim, right? This means that you have to enlarge that parcel to meet the amount of wanted prims. But make sure that the land is on the same avatarname or group, so the total sim prims are devided correctly.
If you need more help on this matter then feel free to contact me inworld. My offer to help inworld also stands for Tony.