It is possible to have a travel catalog. I do this on my Macbook Pro using a 128gb SD card. It means I can then process, edit photos and add metadata etc. When I get home, I import the travel catalog to Lightroom on the desktop PC to combine it with the master catalog.
Here is a video tutorial which demonstrates the use of a catalog on location.
There isn't an easy way to do this. LR is designed as a single user, single computer, image management program.
Some have gotten around it by syncing the LR catalog with one of the online cloud based systems like drop box. You still have a local catalog file on each computer but you sync each system with DB. This will lead to problems unless you store all the images that are in the catalog file on an external drive and connect that drive to each system when you are using LR.
Perfect solution. I too travel with Mac but use PC at home. This keeps the catalog unified. It's a little complex, but seems completely possible. I'll try it on my next travel.