Even if you get suckers to form, you'll find that they are still very much attached to the mother plant and have virtually no root system of their own. Suckers have to get very large before they can be divided off from the mother plant, probably the size that your mother is now.

This is speaking as a landscaper in southern California that deals with these plants all the time and they are growing in the ground here which means that they are even more likely to develop roots, but the suckers don't.

Strelitzia nicholai is commonly used as a large indoor foliage plant in California, where it can do quite well long term if it is treated well. If you have the room for it, I would suggest keeping it as an indoor plant rather than trying to "dwarf" it by letting the first frosts kill the tops from leaving it outdoors in the fall. They are relatively slow to regrow from the roots if they are frozen back to the ground, and as Cagary says, they don't divide all that easily as the roots are massive.

I'd avoid fertilizing them much to try and slow their growth down, and give them enough light indoors so that the foliage doesn't become too floppy. The actual trunks take some time to form, and new leaves aren't really going to increase in height significantly if kept as an indoor plant. Unless you have early spring warmth, this plant is unlikely to recover well from being completely frozen back, although they regularly do so here in northern California conditions, but then again we are USDA zone 9b climate.

Thanks.... I was more thinking to sacrifice the mother plant in the name of getting suckers, but if those will likely die too, that's no good. There is no way this would survive a winter here.

How big would it be expected to get before it began to show trunk? The highest it could be expected to get in my house is probably about 10 feet, unless I move it to a foyer which has a 20 foot ceiling, but not a lot of light. I guess I could just tease the leaves away from the ceiling to make it act shorter than it wants to be.

It might do well in that 20 foot high low light location. I've seen them look pretty good indoors in low light conditions.

I have grown them by a bank of windows indoor,and they are very fast growers with good light. BUT,since they are trunking plants to keep them in bounds is to not pot them up past your ideal size. Eventually the leaves will become smaller and more closely spaced. Continue to take good care of it and you will have a short trunk with half sized leaves or less. Looking much like a Bonsai dwarfed Travelers palm.

They tend not to form trunks as quickly when grown as indoor plants, while outdoors they may start to trunk up when overall height is still less than 10 feet tall. Leaves tend to get too long and floppy if grown with too little light, I would think that the 10 foot ceiling area is a better fit for the plant. If you want to treat it as an indoors in winter and outdoors in summer plant, it would probably do well in a container if not moved directly into sun, but kept in bright shade in summer. Growing it outdoors seasonally would allow you to get more clumping, and you could then cut out the too tall parts leaving the shorter ones to carry on. They tend not to clump and grow babies as quickly under lower light indoor situations.