Talk to your LAME and find out what he's views are for "on condition" engines. When allowing for an engine to continue past TBO, many factors can contribute to the decision. How long since the engine was O/H? If its longer than 12 years then the engine is already "on condition" as far as the manufacturers concerned. Has the engine been topped recently? Bulkstripped since last O/H?
Generally if its running strong, making static RPM, Not burning oil or fouling plugs and theres no excess metal in the filter then it shouldn't be a problem for most LAMES to sign it out
JS