Hello Mark,
In the 1.0.0 pre-release you can enable "experimental formats" in the Advanced tab of PhotoScan Preferences window. This will add "Export tiles" option to the Export Model sub-menu, allowing to split the original model into tiles of fixed size (we recommend to reference model using scale-bars or other Ground Control data, since the size of tiles is set in meters).
It's experimental feature, but probably it will suit your needs.