Certified GKH Gardening Expert
For the seed saving process itself, yes, the same process is applicable in the tropics. Onion varieties that are adapted to growing in the tropics do need different conditions to produce seed in the first place, so the timing may be different: onions respond to day length to determine when to produce flowers and seeds. Varieties grown in the tropics typically produce seeds in response to short days.
You will also need to make sure there are enough pollinators around, since onions are pollinated by bees and other insects.
Check this site for more informantion. The first search result is a very helpful pdf: