Having run full hydro on my crawler for years, I can tell it can be done on the stet, but as others noted, there are issues. I dont know anything about the street legal one...
I have a full PSC kit on mine- I dont know if it is a return to center, but it handles well at speeds, so I will assume it is. I have driven the rig on 44" TSLs (not balanced) up to about 60mph on back roads with no issues. You need to pay attention as there is literally no road feel. It kept the tires where I pointed them. I have only done this when around crawling parks to get fuel (this was in Harlan, KY- road was a 55mph zone)
As noted, if you get in a wreck, expect a lawsuit. Most systems are not street legal. I doubt if you got pulled over by a trooper that you would get a ticket, many have no idea what they are looking at.
Also as noted- without the hydraulics working and no mechanical linkage=no steering- period. I witnessed a hose failure on a road- the driver lost control and the rig flopped into a ditch. Luckily this was around 25mph, but the hose gave no warning that I am aware of, completely blew and with no pressure- no steering.
All that said- why not hydro assist? With 37s, hydro assist would be really beneficial and negates all the problems above- its legal and still retains the mechanical linkage.