Is there a way to make Lion's scrollbars visible on mouseover them? I have started trying the "Jump to the spot that's clicked" option under General System Preferences because I'm pleased by Lion's vanishing scrollbars but they complicate finding the current location before scrolling.

There is however one serious problem with the "Jump to the spot that's clicked" option : You'll never even see your current location in the document, because scrollbars only become visible when selected, which makes returning challenging.

Is there an option or tool to either (a) show the scrollbars on mouseover, or else (b) always show transparent scrollbars.

If not, can anyone point me towards the most relevant Cocoa documentation? I should be able to implement this myself but any tips might save considerable time.
I don't think there is a way to do either of your choices built in, and I don't know of any tool to do them for you. The only way to get this ability in every application would be to have a plugin which is automatically loaded into every application and replaces the default scrollbars. A possible workaround is to scroll just a little bit, so that the page doesn't move much. This will cause the scrollbars to become visible, and they won't hide again as long as either you are moving the mouse on top of that window, or you have the mouse above them. This means you can scroll a little, then move the mouse on top of them, and not worry about them disappearing.

If you want to try to implement this yourself you will probably need to create a custom NSScroller subclass, and find a way to make NSScrollView use it instead of the default class (I think NSScrollView has a private method to create a default scrollbar which you could override, but you will have to find it yourself).