The bump stops idea we used (back in the day!) by putting fuel tubing over the shock shaft. The tubing could be adjusted for length, and by taking 'nicks out of the sides with a scalpel, we could change the 'rate' of the bump stop to be more like the spring assist.
The 'gas' shock Pookster describes we already have, albeit only at atmospheric pressure. The bladder in the top of a lot of shocks acts like the gas piston on top of the oil, its just that we don't put any pressure behind it above atmospheric pressure.
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