Hmmmmmm..... You can run a good experiment here & let us know what happens :o)
I would cut it back by half or even more if the new pieces are planted in a windy location. And make sure that the new pieces are FIRMLY tucked into their new homes. I think that the biggest problem with transplanting or dividing at this time of the year, when grasses are going dorment,would be from the root ball loosening during the cold winter months. On the other hand if you can leave some foliage, it does serve to trap leaves & snow providing added winter protection. Couldn't hurt to mulch a little around the roots, not over the crown.

As long as the weather remains above freezing the soil will remain relatively warm so ... I would also fertilize weekly with a liquid fertilizer with a high middle number and 0 or very low first number. ie, lots of fertilizer for the roots & none for the top growth. OG's do continue to put on root growth at this time of the year so you could help it along a little. Encouraging top growth would be a bad idea - my most tender Miscanthus cultivars do poorly in zone 5 because they don't seem to know when to stop putting out new growth shoots.

Again, you are stretching the envelope - let us know the results in the spring!
A.
ps, topics of transplanting & dividing at this time of the year are of special interest to me. We are in the process of moving house and I have been eyeing a few of my grasses at this late date. I really would like to take them with me ....
I too will let you all know what the outcome is in the spring. (moving from a 5 to a chilly 4)

I posted a reply on the thread titled "Transplanting Maiden grass in fall", detailing my experience with transplanting Gracillimus over Thanksgiving weekend! 2 of the 3 divisions survived with very little pampering and no mulch.

I think they have a good chance of surviving - if you keep them well watered, and mulch generously after the first freeze, this should help.