I'm pretty sure this is what you're looking for.
While I have not analyzed the VCS and its interaction with the Harmony, and current draw on each address line during operation, I will have to say that the Harmony cart is 5 volt tolerant. Not native. And the datasheet of the LPC2101 confirms it. But is being 5volt tolerant an unusual strain on the console? That depends on how the two entities interface. Simply being 5v tolerant could create heat if the tolerant part draws too much current.
Someone will need to add up the current draws and sinks and pull-ups and see if they put undue strain on the 40+ year old parts in the VCS. The LPC2101 was chosen because it had a lot of the right functional features and happened to work with the VCS' 5v bus. But it's really a 3.3V part.
Someone should put the combo on the bench and do a couple of tests. Watching the power supply, watch the rise and fall times, checking for sagging and ringing, checking for incomplete pull-ups and pull-downs.
It is of my official professional opinion that 3.3V parts and 5V parts can co-exist quite safely with no long-term ill effects to either, provided they are interfaced with level translators or buffers spec'd for that purpose.
Some flash carts do this. Others do not and fudge it with direct connect (very bad), or pullup/pulldown resistors and tolerant parts (slightly bad).
Parts operated without level translators may not fail right away, but they do age faster. Anything operating out of specification will have a shortened life.
It is important to note that different parts (of the same number, i.e. 6507, TIA, whatever) can and do age differently and at different rates. So some combinations of swapping things might make you a system totally out of spec and incapable of operating. But put some of those parts with "stronger" parts, and it evens out. And the system works.
And the trick here is to realize that with 40 year old parts some of them have naturally drifted close to their min/max specification. And when you have a lot of things going on, like in a computer, that can all add up. Like a house of cards. One little nudge in the wrong direction and a logic gate fails to trigger on-time or weakly, and the house falls down. Do the address/data lines in the VCS look any different with/without a Harmony plugged in? Inquiring minds want to know!