I'd think infrastructure mode would be easier than ad-hoc, if you already have a working WiFi network that the module's firmware supports connecting to. (The module should be "smart" enough to get on the network by itself once configured. Most of the ones out there seem to even support DHCP. You'll be hitting the PPP server via its IP address so you shouldn't need a dedicated ad-hoc segment between it and the device.)
As for the server, it should be roughly equally easy/hard to do it with just about any UNIX-like OS. (I've used pppd as a VPN server tunneled over ssh on Linux, FreeBSD, and MacOS X, and that's *basically* what you'd be doing.) A really bare-bones module *may* not support creating an outgoing connection (which would be the case if it's designed to network a serial peripheral). In that case you'd have to open the tcp stream from the server end. Otherwise... again, depending on the module you could either simply enable "telnet" on the host machine and stream PPP over port 23, or you could pick an arbitrary tcp port and script a simple "PPP connection server" to run out of inetd that would listen to connections from it. Once the tcp connection is established you'd run whatever PPP dialer/stack that tickles your fancy on the Mac end (I've never used a classic mac over dialup so no recommendations here), and have a container script on the other end run "pppd" with the appropriate flags to assign an IP address and get the Mac routeable.
(If I had one of the doohickies and an old Mac I could write up the exact procedure and write a script, but as every doohickey is going to be different the devil is in the details. But any semi-decent UNIX nerd should be able to get you going.)