does not look like the SAILtoSAIL (siblings) trunk option was meant to do this
Ah ha!... but it was! (at least later, when we realised it couldn't

).
You'll need one of the later releases (can't rememeber when we put it in) but you can define a Sibling link as "unprivileged". That's the one you want.
Normal sail to sail siblings get a concatenated name (like fredgeorge) unprivileged siblings get a name with a funny squiggle in the middle (not on my keyboard so I can't show it here) - fred{squiggle}george.
They appear in the inbound route drop-down for any trunk so you can switch an inbound call straight off to the target machine. It was really developed for T1/E1 consolidation but it works very well for distribution of any inbound DDI (DiD). On the target machine simply define another unprivileged link (back to the first machine) and then create PTT-DiD entries in trunks for the inbound DDIs. Calls will arrive looking pretty much as they do when they come in off any regular trunk. You can then switch them to wherever you want them to go on the local box.
Et voila!

S