I think you totally misunderstood the description. I don't know which version of PLS that you are talking about. In current version, the FAQ says:

"If you do input decimal onsets, they will be rounded to the bottom slice using "round" function (before Feb 24, 2011, they were rounded using "floor" function)"

This means that PLS is not using "rounded down" any more after Feb 24, 2011. This decision was made by discussion.

I also searched word "interleaved". It only appear under "Use HRF instead of Block length". The FAQ says:

"If you input decimal onset time directly, please make sure that your slices must be in sequential order rather than interleaved order. Also, please make sure that your slices must be in ascending order (i.e. 1st slice is at bottom) instead of descending order or rounding order. Otherwise, you must enter integer for the onset time."

If you tried "Use HRF instead of Block length", you will find that this is a very special feature for E.R.fMRI.

If you are not using this feature (by default), events are treated volume by volume (i.e. entire brain), and you do not have option to choose slice (i.e. decimal).

If you are using this feature, then you need to follow the description I mentioned above. Therefore, to answer your question, if the slices are interleaved, it is not supported.