As was explained above modifying the roll to 66 using Ambush is not forbidden by the RM2 or RMSS/RMFRP rules. In fact I personally think that the 66 was deliberately added to the critical tables to be used with Ambush.
I've been thinking about this since the thread came to be, and looking back at the gaming sessions we've had over the decades, it would seem this is exactly why "66" was stuck in the middle of the crit tables. Looking at it with fresh eyes and fresh opinions on the topic, it's given me a new perspective. We always said (i.e.: house ruled) the 66 had to be an UM because it was so powerful but without this magic number, Ambush is only used to increase the crit roll to hopefully get to the next threshold on the crit table. Maybe the 85-93 category becomes the 94-100 category* and gets the better result.
*-I can't recall the exact numbers, just using it as an example here.
The crits can be modified DOWN by Ambush skill and the way we explained it was "maybe you don't want to kill the target, just ambush to incapacitate" so you can modify your high crit to a lower value and hopefully get a less deadly, which we did quite often. Funnily enough, it didn't always work; high crit, but Ambush skill just not good enough to avoid killing the target.
However, we also used the Ambush skill to set up a successful ambush situation, so this is a skill that we got double use from and it added value to DP investment. We removed the 66 kill but allowed it as a bonus for setting an ambush in addition to modifying the crit roll. Again, it was all house ruled, but I think I'll be removing that house rule now.