Not to resurrect an old, dead thread - but back before the SPCS was fielded (and back before the days of "OCP"), I modified my IOTV to use as somewhat of a "plate carrier," and it worked reasonably well, considering.
I removed the soft inserts as well as the elastic cummerbund, and tucked the flaps of the armor carrier back around the plates - it could then be used with plate backers (or with standalone plates).
Without the plates, of course, it was horribly floppy, nor was it the lightest plate carrier, but it worked at improving the level of mobility and comfort of the IOTV and cutting back some of the weight.
In the end, IMHO - if you already have, cannot afford anything else, or are otherwise "stuck" with the IOTV, it's not a bad option - however, if you at all have the option of purchasing and using a dedicated plate carrier, I would go ahead and do that, rather than trying to convert the IOTV into something that it's not.
The only real benefit, I suppose, of this, besides utilizing an existing IOTV is the fact that you can relatively quickly switch back and forth from a "PC-esque" setup to a full armor carrier as your needs/situation dictates.
~Augee