Ungh.
The problem is subspace has always been integral to the plots of Freespace. Subspace nodes are key strategic objectives for inter-system wars; the Shivans were obsessed with controlling individual jump points; the destruction of the Lucifer mid-jump destroyed the node to Sol; Knossos portals can repair unstable nodes; the Shivans in Capella were sealed off from the rest of GTVA space by collapsing the Vega and Epsilon Pegasi nodes... by introducing an alternative, a competitor, a rival physics dealie, you cheapen it, make it less special and less unique.
Furthermore, it's not a natural progression of existing ideas. It's like "Hey, everybody, it's FS3! However, we're not going to deal with the Shivans in this game - nor, in fact, anything to do with them! Nah, this time it'll all be about the Xarg, a completely made-up species that has nothing to do with the rest of the FS universe! It'll be ace!"
From subspace to super-subspace-using-80-juggernaughts-and-a-supernova, yes. From subspace to hyperspace... I don't see the link.
But feel free to go ahead and use it, that'd be fine. Just be aware that it won't really be 'proper' Freespace, like.