What I was suggesting isn't really a spool of pipe but a very short (< 6") section of smaller diameter pipe which would simulate a venturi. The injection point would be in the smaller pipe.
The venturi concept relies upon the fact that pressure will drop when water velocity increases. So by downsizing the pipe for a short section, you will get a pressure drop (more suction) but it is temporary unlike head loss with a valve is. The pressure rises once again once the pipe expands back to the original size.
For example, if you had a step down from 2" to 1 1/4" pipe at 50 GPM, the pressure drops by 1 PSI just due to the velocity change and other 0.15 PSI due to the transition and pipe. The total head loss induced by the step down, 6" of pipe and step up is less than 0.9' of head. To get a 1.15 PSI drop with a valve, you need the equivalent of 65' of 2" pipe with a head loss of 2.6'. So all of the pressure drop with valve comes as head loss while most of the pressure drop in the venturi simply comes from the pipe reduction and head loss is kept at a minimum. The larger the step, the bigger the pressure drop.
But I agree, it may work fine without that but it depends on what the Liquidator actually needs in terms of suction. But given the issue with the Liquidator and the fact that there is less than 0.5' of head loss difference between 5' of 2" and 1.5" pipe, I wouldn't bother with 2" pipe. It may end up causing more problems than what it is worth. I would keep the pipe you have and just see if works. If not, then we can discuss other options.