guess what I'm asking ultimately is if my MPPT charger received current from this, would it treat it any differently than if it were coming from a solar cell?
No, it would treat it the same.
But, the issue is whether the engine/generator combo will appreciate being treated like a solar cell!
How a permanent magnet alternator, or a 'regular' alternator with an electromagnetic field controlled by a regulator, would act in this scenario is not the same. But you linked a permanent magnet generator so I'm going with that.
What would happen is that the mppt would see the 'resting' voltage of the alternator output, which would be determined by a balance of two forces: RPM, and load on the output circuit (zero in this case). I believe if you spin up a permanent magnet alternator while the output is open circuit, the voltage will rise to a very high level, possibly enough that i'd be worried about the breakdown voltage of the rectifier diodes. But, seeing as the rectifier is external and easily inspected, this should be easy to rule out in initial testing as a possible issue.
As the mppt starts to allow current to flow and tries to find the 'maximum power point', it would allow more and more current to flow from the alternator into the mppt. Since a permanent magnet is not 'adjustable', the alternator itself will have no reaction to this, but as the current flowing out of it increases, the power requirement for the engine increases. So, the governor on the engine will have to adjust the fuel delivery to try to maintain the 'governor rpm' setpoint, probably ~3600rpm. If the max power the alternator can produce is less than the engine can produce at 3600rpm, the tone of the engine will change as it 'loads up', but it will not really slow down and the mppt will settle at that point and it should all work fine.
But if the max output the alternator can produce is more than what the engine can produce at that rpm, the engine will 'bog down' and lose rpm until the MPPT realizes it has 'gone too far', and starts going back the other way. Depending on how that plays out you could end up in a constantly repeating cycle of engine bogging down and then revving back up, or if the mppt 'loads' too quickly and doesn't 'release' quickly enough, it will simply stall the engine.
So the safest way to attempt this would be to have an alternator whose max possible output is comfortably LESS than the engine's max output, so that the engine could adapt to the load by changing its fueling (and throttle opening, if it were a gas engine) but not actually be pulled substantially down in rpm. Because once you exceed the maximum available power of the engine by ANY amount, it WILL slow down until it stalls, and because the engine's power output drops with rpm, the amount that it 'falls short' increases as its speed drops, which means if the engine starts to bog and the situation is not remedied almost instantly, it will quickly escalate to a stalled engine.
So if you take a 2.2kw engine and hook up a >2.5kw alternator to it (considering efficiencies it is probably over 2.5kw on the input side), and then an mppt to the 2.5kw alternator, it
might stall the engine. If it doesn't, it is likely to go through an irritating cycle of bogging the engine down and then letting it rev back up every time it tries to recheck the max power point. If that's not hurting anything and it doesn't bother you it's technically fine, but i don't see that scenario resulting in the engine just happily purring along at a steady rpm and load.
And now the part i have no idea about... 3-phase ac rectified through some diodes is still pretty wavy. If you feed that to an mppt, is it going to actually react to the constant small changes in output and be 'seeking' mppt constantly? And if so, is that harmful? I don't know what amount of waviness you can feed into an mppt without causing issues.