it restricts it to a switching-only use-case then
Which switch(es) are we talking about? You don't say, either here or in
the other thread. Lacking that detail, let's look at the one the thread-starter is using, the
netPower 16P, which has a lowly single-core 800 MHz processor. That's not much for routing.
If you need L3HW to get acceptable performance on such low-end hardware, it's a sign of this very problem: you've picked underpowered hardware. It's a nice feature to have to keep CPU usage low, and it may be essential at the high end where no more CPU power is reasonably available, but to be utterly reliant upon it otherwise seems like you're pinching pennies somewhere.
Lacking details of your reason to want this combination of features, I'll offer a few alternative suggestions blindly:
First,
router-on-a-stick: Add a proper router to the system — being one sized to carry your traffic at line rates — then configure bridge VLAN filtering on the switches to distribute traffic in hardware at line rates per the VLAN tags assigned by the router.
Second, if your purpose for MLAG is bonding switches together for manageability, the
Controller Bridge and Port Extender feature may do what you want.