Substituting maltose for barley malt syrup in bagels

I would expect that substitution to work fine, in a 1:1 swap.

Both Brown Rice Syrup (what you're calling Maltose) and Barley Malt have a high amount of maltose-the-sugar, plus an assortment of other sugars. Both are pretty close to pure glucose in their actual sugar content, while at the same time tasting less sweet because of their composition and some of their minority content.

Barley Malt is going to be a bit "darker", with more "roasted" flavors than commercially manufacturer brown rice syrup (although artisan traditionally made brown rice syrup would be very similar). So it won't add quite as much flavor to the bagel dough as barley malt will. However, speaking from experience, you can make bagels using white sugar instead of any kind of malt, and they still come out fine.

There's one exception here, and that's pumpernickel bagels. These dark brown bagels depend heavily on the roasted flavors of dark barley malt (in either syrup or powder form). I wouldn't try substituting with those.