This is an excellent question!
First off, not all Drosera crosses are sterile. Between members of the genus sharing a common karyotype, e.g. 20N (this particular number is common amongst the South African species) fertile crosses are certainly possible - just as in the case here with our genus Sarracenia. Many complex crosses can be expected where species sharing a common karyotype have overlapping ranges. Once there is significant variance in the"N" there's not much chance of any successful pollination.
The petiolaris group is a good example of how these complex crosses can happen. Look at what growes have produced in the last decade alone, not to mention what is to come
What limits these crosses in habitat is isolation; the species are widely divided - usually by desert like hell on earth. There is considerable room to further reduce the number of species in this section of Droseracae for some aspiring PhD! I suspect many of these so called "species" are not (and the same for many pygmy's). Rangeschange over time...lots of time (!) and connections get lost in million year drifts. Species or hybrid is a very opinionated thingy.
I suspect many of the pygmy species are capable of making fertile crosses between species, there are many already in circulation. Seed is not often produced in quantity, but it happens.
Not to mention the inevitable crosses possible amongst many of the South African species what can't keep their hands off'n each other....scandalous wanton crossing anywhere the ranges are sympatric. What a mess! (and a word to the wary if you collect these, when different species share a common flowering (antithesis) they may get it on when your not chaperoning them, and the seed you *think* is Drosera dielsiana may in fact be some *******o result of all this. Tsk tsk. You'd need to be crazy to take on S. Af. taxonomy. Do yourself a favor if you think you have academic interest in this and DON'T!
A simple websearch for "karyotype numbers for Drosera" should allow you to find commonality in this regard between different species, and could suggest many interesting experiments.
Refer to the "article" section for Ivan Snider's fine article on pollinating proceedure. (Yes, I deliberately spelled that name wrong, payback for the Listserve drumming I got over my publication of Drosera 'Rhodesian Beauty' in the CPN, harrrrumph!). It's a fine article though.
Please experiment! You never know what beautiful results wait for you and the world!