There is no equivalent to ENVI_GET_SLICE for columns but you should only need to rearrange the data request a little to get a column.  For example, if you use ENVI_GET_DATA, you could set the DIMS to get a single sample but all lines, which would return a column.  If you use ENVI_GET_SLICE, you could use the XE and XS keywords to specify a specific column.  For example, xe=2, xs=2 would only get data from the 3rd element of each line thus returning the 3rd column.