Hi Rip,
A few weeks back, towards the end of the British summer, I was on the beach with my wife, my teenage daughter and her friend. The girls were messing around doing acrobatic stuff (or at least attempting to do so) and they challenged me to do something 'impressive'. Not being the acrobatic sort and not being keen on sending myself to hospital proving it, I offered to do a plank with wife and daughter lying on top of me. I succeeded in doing this for about 30 seconds, which was deemed impressive enough for me not to get pressurised into trying something which would put me into hospital for the night.
And from a training perspective, this plank actually felt useful - it used the abs in their primary stabilisation function and was hard work in a short period of time.
Given it's not practical for my wife and daughter to come to the gym with me three times a week to lie on my back for a plank or two in the final 5 minutes of my workout (and I've yet to work out a safe way to add my bodyweight to my back in the absence of family members), this got me thinking about other 'useful' ab exercises as alternatives. [And by 'useful' I mean work which could help the main lifts - in the same way that back extensions are a useful assistance exercise for the opposite side of the c**e than the abs.]
On the forum, I've seen recommendations for planks, ab-wheel rollouts, even dragon flags and you've mentioned the 90 degree sit-up contraption (which don't have in my gym), and I've also tried L-sits (which seem like a decent use of 30 seconds x2 at the end of a session) and I just wondered what your thinking was on 'useful' ab work these days, and whether this has moved on at all in recent years?