Some more updates, including basic FPTest support.
Screenshot shows the new Gutter within the FPCUnit information window, next step will be to add it to the IDE (as long as the original file is untouched).
Biggest remaining task is the line-based coverage information. I've been looking into the debugger system, but it's not helping a lot yet. I haven't found out how to access the main debugger yet (it seems to be in main code, not in a package), the DebuggerIntf package does not seem to return access to the current debugger. Enhancing one of the included experimental debuggers would be an approach, but lazdebuggergdbmi for example isn't even true to the actual line. That might be the only approach possible though... enhancing an existing experimental debugger, replacing the Run method with a loop of Step Intos, for each step, check the callstack for one of the coverage methods, and if so, log the line access (the callstack is necessary to have the information which test has covered a line).