There is always the possibility of mixing slow release fert directly into the bonsai soil mix at repotting time (early Spring).
Is it a wise thing to do? I keep wondering about one or more of the little pellets exploding inside the rootball and doing real damage. In short, is it safe? Does it work?
For the most commonly available brands of coated, time release fertilizer, the rate of release is temperature dependent. My climate varies from -32 C below freezing in winter to +32 C above freezing in summer. This is pretty extreme. IF you can find a coated fertilizer designed for you typical temperature range, then it is a good thing. Do not overdose your soil.
I had a pine that looked like it needed nitrogen. I top dressed with a good dose of a coated fertilizer. A month later the tree still looked chlorotic. This was spring, weather was running 5 C to 15 C, at the warmest. It was unusually cool weather, but it did not occur to me to take the cool weather into account. So a gave a second "heaping dose" of fertilizer. Finally the prevailing wind shifted, and summer kicked in. Daytime highs of 30 to 35 C. Bang, tree turned green for a day or two then suddenly turned the most frightening yellow and gray. All the new growth died. The tree was being poisoned by excess fertilizer. I had to do an emergency repot. Put the tree in new potting mix with zero coated fertilizer. The tree took 2 years to recover. The third year it was finally growing normally.
Result, I still use coated fertilizer, mostly as an additive to pots with young trees that I am encouraging to grow rapidly. You must keep in mind that coated fertilizers have their release governed by temperature. You can use it to your advantage. It can also be a disadvantage. You just need to keep this trait in mind.