I don't care if I have to add libraries or build whatever on my end. I'm just hoping since OS X has the foundations of Unix that there's some way to run a Linux or BSD client. Specifically, we need a VPN client that's Mac compatible, and ShrewSoft.com has one for Windows, then another for Linux/BSD, but not for Mac. I'm wondering if I can hack-n-stall it on OS X Lion.
Though it is generally possible to port software from Linux over to a mac, there are some exceptions. Software that requires or links into the kernel generally need to a lot of work to be ported, since they require building a kernel driver. VPN software almost always falls into this category since it needs to hook into the networking stack.
I know this seems counter-intuitive, but have you thought of running the Windows version of the VPN client, not in a virtual machine, but with Wine for OS X ?
Wine can be a bit of a hit-or-miss solutions at times: some programs won't run no matter how much you cajole them, many run more solidly than they do on Windows. Especially if, as is often the case, the Windows client is more mature than the Linux/BSD version, it may be something worth taking a look at.
I have never tried it but it seems possible with Fink: