3.5V is a 'odd' state of the USB bus - there is no guarantee that stuff will work - even if the power supply stays at 3.5V under load.

If it did - then there is a chance that some USB devices may work - but most will not. The USB voltage is 4.5 (IIRC) minimum.

There are a couple of issues with regards to using USB devices or hubs from an unpowered host.
The most obvious is there is no power - selfpowered stuff will not work.
Second is that even powered devices sometimes look at usb to see if there is a host there that they should negotiate with.

One printer and one hub I have will not talk to an unpowered host (lexmark printer, and Belkin Tetrahub).

(unpowered may or may not behave as 3.5V does).

99% of cheap external powered hubs will not care.
There is a very simple way to test this.
Get a standard USB cable.
Chop it open.
Cut the red wire. (being careful not to short it, or it will explode! (or not - however - insulating the cut ends is a good plan))
Try plugging stuff into a normal host.

(try with a few cables you don't care about first, if you're unsure.)