I haven't found any thread anywhere regarding AI using land transports.
I used Steph's editor (or maybe it was Quitillus') and checked the box "Transport" in Strategies/Air without defining it as an air unit (still land unit). I first tried to check the "transport" box in Strategies/Sea, but although AI loaded the transport with units, it would always be built in a coastal city, and would never leave the city square.
Another important thing to remember when using this trick is that Firaxis' editor will remove the Strategies/Air/Transport-flag from land units whenever they are edited. In an un-edited unit it will strangely enough indicate the box as checked (even though it is greyed out). Quintillus' editor seems to struggle with these settings when loading a biq edited by other programs, as the upgrade paths sometimes vanishes. I haven't tested this trick thoroughly in his (otherwise excellent) editor and don't know what causes this.
The settings I use in the land transport units have both attack and defense strength in addition to bombard range of 1 (and of course transport capacity > 0 and movement of 2 or more).