zbawic, yes you can! A back pack sprayer will be more convenient but any of the other type will do. The flow of water delivered can be changed by simply removing the nozzle at the tip of the wand. From there on this flow is dependent upon pressure which in turn you can control by the hand pump. The advantage of the back pack is that you have continuous control. Indeed if you remove components like the filter and automatic shut off etc., you can water the plants by gravity alone. With the other types one has got to stop and pressurize the equipment; then at first the flow is rather high and then decreases until it is time to pressurize it again.
You must have seen the caution many times on this forum but I will repeat because old habits die hard;
DO NOT use this sprayer for anything else EVER!!!

Can you recommend a brand that is high quality? I've looked around various stores and don't remember seeing a backpack version, but I probably wasn't looking for that type so it was probably right under my nose.

Yes you can use these for watering and the nozzles on all the ones I have bought are adjustable. However the thin stream will not put out volumes of water. The flow will be in the shape of a stream, but not lots of water like a watering can. I've bought 2g ones and smaller hand-held ones and they all work well. Expect to replace the seals every 2 years. Just buy a maintenance kit for your model.

I have one for weed killer for the lawn, one for fungicide, and one for brush killer to kill all those pesky mulberry trees that sprout up like weeds.

If you buy one for water be sure to label it "H2O only" so you don't put chemicals in it.