I am not aware that Lycoming specifically prohibit leaning - In fact at high elevations and/or high temperatures you will NEED to lean to achieve maximum available power for take-off.
The EDM that IO540 mentions is a fab piece of kit - specifically because the probes are positioned in every cylinder - not just in one. That means that as you lean you can see which cylinder reaches peak first thereby reducing any tendency to run too hot.
I used to fly aircraft with Continental IO 550 engines. That engine was specifically set up to run with very wide throttle settings (full in many cases) and various power settings both lean and rich of peak (by 40 deg F). Interestingly, peak EGT was not approved for any length of time due to this being too hot.
By not leaning you are actually increasing wear on the engine by causing excessive lead build up on the valves which in turn fail to seat properly.
It is also worth noting that most of our piston engines are fuel cooled to some extent (as well as air cooled). The limitaion on leaning your engine may occur way before 40 deg rich of peak if the cylinders fail to cool adequately thru lack of fuel.
I am therefore a great exponent of multi-probe EGT and CHT instruments as well as accurate fuel flow instrumentation. Sadly the CAA make it rather expensive to pursue installation of these devices as far as full certification and approval (and then bang the drum about being a safety regulation group!)