We'll be looking at the lack of a zoom key combo for a future update, we do consider that to be an oversight in our current shortcut keys. The current version of ArtRage does support Space to drag so you should be able to return to your previous tool without specifically selecting it, but if you're using an older version you might not be seeing that, so it'd be worthwhile checking you are using 2.2
The 2.2 Full Edition is a free update via the artrage updater ( you can get it from our download page if you're running an older version - www.ambientdesign.com/artragedown.html ), or by downloading the latest installer by entering your details here www.ambientdesign.com/artreageinstaller.html
The 2.2 Free Edition can be downloaded from www.ambientdesign.com/artragedown.html
If you're not getting the behavior I've described abrove in 2.2, just fire me an email at firstname.lastname@example.org and I'll see what might be causing the problem. Hope that's of some help.