Just a quick heads up before I start on the ellipsoid form factor alternative: I spoke last post of the efforts for data archiving for possible open-access purposes. Shortly after that post, this news appeared. It seems we may be heading (more rapidly than I thought) towards an age where we have to make data public, which means archiving with metadata and storing in an archival format. I do hope (Matlab) writing and reading functions for NeXus format files become available soon.
Regarding the ellipsoid form factor, I have mainly been using the ellipsoid adaptation to the Rayleigh sphere scattering function. However, this function requires integration over all orientations (see f.ex. equation 3.46 in the SASfit manual). This becomes very time-consuming for use as a fitting function if you furthermore would like to integrate over minor- and main-axis size distributions.
An alternative form factor appears to have been published in a light-scattering paper in 1969  by Beattie and Tisinger. Although this procedure requires an iterative approach, it may prove to be faster to use than the aforementioned method. Efforts are underway to implement this method. Assistance, as always, is highly appreciated.
 Beattie and Tisinger. Light Scattering Functions and Particle-Scattering Factors for Ellipsoids of Revolution. Journal of the Optical Society of America (1969) vol. 59 pp. 818