Kind of leave it open to interpretation, don't they? /w3tcompact/icons/smile.gif I am no expert on this, but I'll give you my opinion.
Aircraft engines have some similarities to tractor engines in that they both operate at (relatively) low RPM. That said, standard procedure with aircraft engines is to break them in at maximum operating RPM as long as they stay within nominal operating temperature.
The major difference between aircraft engines and tractor engines is that tractor engines are designed to operate at lower (still) RPM, and are optimized for a flat, broad torque curve.
Bottom line, is that you don't want to jeapordize your long-term torque-producing capability of your engine. So, after a few hours, I would operate at a compromised RPM as you suggest, maybe a "bit low" for true PTO speed, but a bit above the very low RPM that might seem appropriate. So, I would operate in the 2200-2400 RPM "range", and keep the HST in low range. That way, you're staying below maximum RPM, and you're not putting any undue stress on anything either.
The GlueGuy