Portable Air/Fuel gauge?

Yes, there are several after-market versions of "clamp in tail-pipe" O2 sensor mounts. You should find some in ebay browsing. I think they are particularly popular with the "Jap tuner car" crowd, since no welding required. I think all of them work only with a wideband O2 sensor, since those can be installed anywhere (as long as not so near the end they see atmospheric air). Wideband O2 sensors have an internal heater and don't rely on the exhaust to heat them. Regular O2 sensors must be mounted close enough to get sufficient exhaust heat (the heater in those is just to speed warm-up). One issue with wideband sensors is to never run them long without being connected and powered, otherwise the internal passages can clog with soot. I recall you can read more on the megasquirt site.