Howdy
I would suggest that you should use the TER to render your lines.
Since your TileEntity stores the position of the next marker, just draw a line to the next position (relative to the position of the TE being drawn, of course). Very simple, very scaleable. renderworldlastevent should be a last resort I think.
The only likely issue is that lines will disappear when the TE is unloaded (goes out of chunk range), which may become very noticeable if the markers are far apart from each other, i.e. you are close to one of the markers when the next marker goes off the edge of the chunk limit.
You could probably work around this by drawing to both the previous and the next markers; if necessary keeping track of which lines have already been drawn during that frame so that you don't draw the same line twice.
-TGG