Sorry to bring more things, to consider, Michael.........but your different plants may differ a lot less, than you think. Most likely with Anubias 'nana', they come from Tissue Culture..........which basically means all 100 milion plants are alike. I see "types" start to show in standard 'nana', though, so if you choose plants, that looks a little different in leaf-form, you are likely to get better polination.
Actually 'nana', 'petite', 'bonzai', 'gold' and several others are also just such "types" of the original Anubias barteri........and should therefore be very capable of cross-polination, bringing unpredictable offspring.😱
Stem-plants would generally be easier, to get to produce seed, I might add.
- like allready said many flowers don't "mature" their male and female parts at the same time ( this partly prevents selfpollination in nature) and some even has ability to "wait" for the right weather. You'll just have to mess about with the tiny paint-brush in as many flowers in as many conditions as possible 😉
I once did a study on pollination and cross-pollination in Passion-flowers, and found that several "sterile" types was only male or female "sterile", and several others was not "sterile" at all, given specific weather (full sun, early early morning, staying sunny all day), but would fail pollination if only sunny morning, but rain later (likely due to pressure in the air!!) Just to show how advanced some plants can become......
- it's a fascinating world, though. Full of surprises..............go explore !!!
Mick.