I couldn’t resist, so I had a quick look.
I checked out the kernel code. Looks pretty straightforward for the rk3588 SoC. Just kernel config and DTB updates needed to enable sleep.
On the rk3588 adapter (reform2-rk3588), the SOC_WAKE net is fully routed: it connects J1 pin 22 to U1 (reform2-som-connector) pin 145, whose function is GPIO1_IO3. Confirmed via netlist export, and schematic inspection. So when I get my Reform Next in the mail I can start testing that board. 
I also had a look at the RCM4 schematic as well. That one doesn’t look so good. IIRC the raspberry pi cm4 didn’t have a sleep state.
The RCM4 board carries the same signal called SOC_WAKE, and it appears twice. On the motherboard-facing connector (reform2-som-connector), it lands on pin 49, whose stock name in that generic connector definition is CSI_P2_D1_N, a MIPI CSI camera lane, repurposed here. So on the RCM4 side, this pin does carry a real signal (whatever the RP2040/system controller drives for wake).
Sadly, on the CM4-module-facing side, that same SOC_WAKE net terminates in an explicit no_connect marker. The just means we need something to wake the rk3588 up, and unsurprisingly the board doesn’t provide it for us.
I might be able to hack something together with USB or soldering a giant button to one of the SOC_WAKE traces (j/k), but I’ll probably pause for now and pick back up with the Reform Next.