I'm planning on making a wall 6 bricks tall. The wall will have some vertical clips coming out to clip to a vertical pole. Unfortunately, the only 6 or 7 length bars appear to have the little stopper knubs on them and I think this may interfere with the design. Is there a way to connect two 3/4 L bars together?
You might want to look at Flex System tubing. It comes in a variety of lengths (including 6L and 7L) and doesn't have a stopper.
For what its worth, this is the technique that Jamie Berard used to create the downspout on the apartment in the Pet Shop:
You may want to try Hose, Rigid 3mm D. 6L / 4.8cm or 7-stud long Hose, Rigid 3mm D. 7L / 5.6cm
Rigid hoses are the same diameter as bars you've mentioned. However they are hollow. But they are hard so they stay where they should when attached to clips.
You can also look at other lengths available on Bricklink.
That might be a bit too obvious a solution so maybe you have already considered that and discarded it for whatever reason. But if you have enough clips (or can increase the number of clips easily), you might not have to connect the 3L and 4L bars explicitly rather than just clipping them in next to each other in their own clips and living with the fact that they're not a single element.