There's always pfSense as an alternative to m0n0wall. I run many of those under VMWare.
I chose it for its easy multi external link capabilities, after I gave up on Linux for this and was pleasantly surprised by its ease of use, stability and huge range of features.
It is nearly bullet proof as I discovered when one of a customer's VMFS died. All the other VMs fell over immediately but the pfSense router carried on running without its "hard disc" for two days before I replaced it. Internet access downtime was 2 seconds as I cut it over. Admittedly the web interface vanished but the routing, VPNs, firewall etc carried on running.
As to OWL, its a Linux distro so it will have no problems with being a VM - that's the whole point of virtualization. You might have to select "Linux other (64 bit)" but my many Gentoo's run happily like that
Why on earth should the devs even think about VMWare, HyperV, KVM or whatever - that's your job! Apart from considering making the guest tools pre-packaged what should they be doing? I doubt they care whether you spec your boxen from Dell. HP, IBM or PC World so why should they care whether it is physical or VM?
As to asking about RAM requirements - I'd suggest (without even having looked at it) >=256Mb depending what you do with it. I've no doubt that fact is covered on their web site. If you are using ESXi and not just playing on your home PC then the answer would probably be "who cares, RAM is cheap as chips"
Go on - try it, I might even do the same.
PS You have a 5 digit /.ID. Have you been moonlighting on other OSs for the last 10 years, asking such questions 8)