"Strong pass" systems are classified as Highly Unusual Methods, so most players never worry about them. My understanding is that they are not particularly overpowered (otherwise all the pros would have switched to them) but that one needs highly detailed defensive methods to defeat them and it's not worth memorizing these methods for the casual player. I am curious what these methods look like. Has any team ever played a Strong pass system at the national or international level, and what sorts of defenses did their opponents come up with?
Strong Pass systems are very rare, and as a Highly Unusual method you will never encounter one outside extended team play or without notice.
In extremis, adopt your Strong 1C defence by substituting a 1C call for a double of a 1C opening, and warm up the Double cards if opponents open a fert (any call besides Pass that is definitely limited) instead.
Anecdotes and alternatives from BridgeBase.com:
When I met it years ago, I played that a One Spade overcall said "I am present at the table". The next player bid 1NT, alerted as Stayman.
"Need" is a strong word, but it's always a possibility to consider making your "opening bids", especially in the majors, be more akin to overcall strength than full openers. This is tougher for the minors because you might need to make convenient minor full opening bids and it can be harder to distinguish them from overcall-style openings.
If they 'open' a pass bid as much as you can as fast as you can. If they open a fert be prepared to double them lots with balanced hands and your overcalls are more like opening bids with shape. If they open with something else then use your generic meta defence and bridge principles.
Use an anti-fert over the fert.
1H = 0-8,
1NT = 0-10.
Its a lot of fun.
Much of the time your specific defence won't be against the strong pass itself, which is relatively easy to defend. In fact the pass itself can be a weakness of the system as your side can come in before the opponents have described their hand properly.
If the pass is actually "super" strong you can defend it like a strong club (above answer). If the pass is medium, so they open on either properly strong or weak hands, you should treat pass as 0-no-trumps so you can come in like you would over 1NT but a level lower. And of course more frequently as it is less risky. (In a typical strong pass system, they like to open more often than not, so pass might be 13-15 with opening bids 8-12, and usually one bid showing 0-7 known as the "fert" bid, often 1Spade, and 1NT or other bids showing strong hands).
What you have to now handle though are their opening bids. These are no doubt the strengths of the system when they open the limited 8-12 range as they open a lot and show their hands reasonably descriptively. You need to also be constructive whilst being able to compete. Be prepared to reach par on these. Go plus when the hand is yours. Compete to the right level. This is your hardest area so probably prepare it well.
Answer above suggests how to handle the fert bid.
I'm pretty certain Strong pass systems were used in the 1991 Bermuda Bowl, held in Yokahama, and won by Iceland.
If you play in ACBL sanctioned events the forcing pass is illegal per their GCC. since they banned it, it must have some merit. as a Precision player, this system was the forerunner for various Precision systems. unfortunately I'm unaware of any local events where it is legal.
for what it's worth, the GCC also bans the popular in Europe & UK multi 2D