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I am afraid the answer to your interesting 'mind-teaser" query about a stack or, for that matter, any piece of medical equipment that has a leakage current higher than that allowed, has to remain a definite "no it can not". Even if on the IPS, and it is almost certain to be so, it reads well below any danger level of leakage current without the isolation transformer present. Immediately though, there is one obvious very good reason why it should not be allowed to be a 'yes' anyway, and that is purely due to the possibility that the stack could be wheeled/moved into another area that is not isolated, and then it would be in violation of safety standards if the Biomeds were not informed in time to adapt it, or worse, not informed at all.
However, there is a more subtle scenario that needs to be examined. The answer lies in a scenario where there are two different pieces of equipment in an IPS area, each of which has a single-fault condition. (Safety limits for all equipment being based only upon a single fault condition occurring in that piece of equipment under test). Let us say that one piece of equipment in the IPS area has accidentally got one of it’s internal main input power lines (let us say L1), shorted to the chassis due say to insulation breakdown. The important thing to recognize here is that now all the equipment in this IPS area is no longer isolated from the secondary side of the isolation transformer i.e. the two power lines L1 & L2 (live and neutral in a non-isolated system) are now referenced to the area earth and the IPS system has reverted to a non-isolated system.
At this point, this single-fault condition would only be a minor problem as having one power line connected to the area earth is, of course, just like having a neutral connected to the earth. In this case, the only thing that would happen is that the isolation monitor should indicate, by an audible alarm, that it has detected a lower-than-normal impedance between one line and earth. This would not cause the power to be tripped but just be a warning to the operators that something was wrong in the system and it needs checking out ASAP.
(If we were to continue to follow this further, it would become obvious that there now is a potentially very dangerous situation if this insulation break-down is not discovered soon. If the other conductor (L2), either in the same, or any other, piece of equipment in the IPS area has a similar case of insulation break-down, consequently shorting to it's earthed chassis, and since all machine earths are still present (and therefore connected together) in and IPS system, there will be a major line-to-line short and a CB should trip, of course. This is one of the minus marks where IPS systems are concerned).
However, let’s now assume that the staff have heard the alarm produced by L1 in the faulty piece of equipment but decided to mute it and carry on with their procedure, and combine this single-fault scenario on the one machine above with your stack (minus transformer) that we will say has a single-fault condition whereby the main power feeder cable has it’s earth-wire detached inside the mains plug going into the wall socket. (This could be due to the staff always pulling the plug out of a socket by the cable alone). Now attach the stack system to a patient on say an operating table. Due to the excessive (over-the-limit) earth-leakage current in your transformerless stack, we now have a potentially dangerous situation waiting to be exposed. If say the patient’s arm or hand touches a grounded point through say the frame of an earthed bed, or chassis of another piece of equipment nearby to the patient, all the leakage-current from your now non-earthed stack has the potential to pass through the patient to earth via any of these points. Since the system earth is now connected indirectly (via L1 in the other piece of faulty equipment) to the secondary side of the IPS transformer the likely results are now no different to those that would have occurred if this had been a scenario on a non-isolated system, i.e. if it is a cardiac patient on the table then your stack might be mainly responsible for an RIP situation?
Hope this clarifies things.
As to the second part of your query, could you please clarify your question as it is not clear as to why you should wish to prevent the use of stacks in either situation and which type of supply in particular you wish to avoid using the stack ie non-isolated or ISP.
