As I mentioned I doubted that seals would be part of the problem in using a PW pump as a hydraulic pump. because as you pointed out Viton seals are used in applications all across the spectrum. any rubber seals would be a durometer of 80 or better Ceramic seals have been used in some Sunstrand piston pumps for years Rexroth Vickers Parker Charlyn all have and do use ceramics in various models of their pumps and motors.
the Cavitation issue is easily solved by having the reservoir above the pump with an oversized feed line. or a simple low pressure rotary vane, centrifugal or gear pump as a charge pump to feed the piston pump. even a salvaged power steering pump from automotive would work well for this.
Building a water jet system using a PW pump could most certainly be done with pressure intensifiers I would think the productive output would be severely limited due to the large volume loss for increasing pressures from a modest 2-4000 PSi up to the 20,000 to 100,000 PSi normally found in water jet systems but again doable by paralleling 2 or more Pw pumps in a system using back pressure checks to prevent reverse flow between the pumps.
here is a 6 pump power unit I made out of individual 3 Hp pumps
I've used these pumps to make power units ranging from a singular stand alone unit all the way to having as many as 10 pump motor assemblies mounted on a single tank. The advantages of doing this was rather than having a single 30 HP motor pulling a 20 gallon pump when only 2 gallons were required or wanted for flow instead of having a variable displacement pump with an out flow of 2 GPM being powered by a 30 Hp motor when only 3 Hp was required for the amount of fluid being moved the savings in electricity usage was 10 fold As flow requirements increased or decreased the motors would automatically come on line or drop off line Also greatly reducing the inrush current load. another advantage was if a single pump or motor were to ever go bad the whole system was not shut down it could still be operated at a diminished capacity if need be. Where as if a single large pump motor went down everything was shut down until repairs could be made.