dumber than a box of hair said:...and probably wouldn't accomplish a damned thing. Don't forget the FCC's current position on moving AM directionals: You move, you lose. It's called "ratcheting," and it applies only to nighttime DA operations. Basically: If you move a DA-N, you must reduce your signal in the direction of the station(s) you're protecting. Moving it to the west, when the station they're primarily protecting (KNBR) is in the same direction, won't accomplish very much, if anything.
I know about the ratchet rule. And I think it also applies to days, but days are not an issue in this discusssion. Nor is KNBR to any real extent. Three stations that definitely would be at issue (and there must be several more) are first adjacent CINF and co-channel WPTF and CFTO (if those are the current calls).
However, the killer turns out to be CFDR which were the calls of the station that was on 680 in Dartmouth (Halifax) NS before it moved to 780. CFDR (or maybe it was known as CKXX in its 780 incarnation) has now either moved to FM or is about to do so. Nevertheless, the 680 allocation in Dartmouth remains internationally notified and that kills the idea of WRKO moving to Ashland at night. CFDR on 680 took advantage of the shallow minimum at 75 degrees in WRKO's night pattern. It's unclear to me why, when the US IA channels were opened to Canada, CFDR didn't move to 670 rather than 780. AFAIK, the protection requirements on 670 and 780 were the same (Chicago on both channels). But since 670 is first adjacent to 680, the 680 allocation would have had to be deleted. CFDR had been on 790 before it moved to 680; maybe the idea in moving to 780 rather than 670 was that some listeners never gave up associating the station with a frequency in the high 700s. Anyhow, to put any sort of decent signal into Boston from Ashland, WRKO would need to use a relatively narrow pattern centered along the (90-degree) axis of the five towers. Such a pattern would deliver a HUGE night signal to Dartmouth/Halifax and would render 680 unbuildable there. It is irrelevant that nobody wants to build on 680 there and that they could build on 780 instead if they really wanted to build a high-power AM in Dartmouth or Halifax. 680 in the Maritimes remains internationally notified and it must be protected, which means that a WRKO move westward would not work.