I cannot say much about QRH procedures here, since I am the guy in the cockpit with grease on his hands:O . A 757 driver can give better answers.
As for 757 wheel well fire, fairly simple. That same manual says there are two loops (one each wheel well) with sensors that trigger when temperatures reach 200°C. There is an EICAS message "Wheel Well Fire" and a "fire" light just underneath the light "brake temp" and next to the three greens. At the same time the master warning signal and lights go on. There is also a test button at the lower right pedestal corner.

Surprises me that brakes can go to 500° without warnings, and well temps can go to 200°. So there must be something requesting the wheels to stay lowered. In real, I've never seen a 757 brake going above figure "2" on EICAS.
I looked up some cockpit pictures on the net, and almost every airline has the brake temp option installed.

On the classic if the brake temps stabilize in the red band (>700F) or the brake overheat light comes on (about 700F), then the gear should be lowered to put the temps back in the green band.

On the CX and QF fleets the A330 have no fire detect sys in the wheel well either.
The 300 deg C brake temp discussed in this thread is related to a performance limitation for an RTO.
The reason this "hot brake" ecam message can be generated in the air and on the ground could be for crew info to action via FCOM etc.
EG a dragging brake/s on take off roll raises brake temp above 300 deg, after flight phase inhibit for this message is removed "DING" ecam msg HOT Brakes . I dont have an FCOM in front of me but I suspect the gear would be lowered until brake temp fell to an acceptable level
:ok:

I will haveto dig out my notes on the Boeing wheel well detection but memory says single loop "graviner" detector ????

Virgin Atlantic has fire detection fitted to its entire airbus fleet, single loops 340-300, double loops 340-600 (one loop either side of the keel beams for the centre gear). Took both Airbus courses last year and there was no mention of Fire detect being fitted as an option - TPIS, thats an option.

With that type of equipment installed, do you know if the ECAM requesting lo lower the gear in case of brake temp above 300C is still active, or the corresponding brake temp value is higher for such an ECAM ?

My notes don't give a temperature threshold i'm afraid, after all we fix them, we don't fly them! FCOM might be usefull there. As with most Airbus systems, it will only give a fire message if both loops, and therefore all four LG Bay detectors detect an overheat in their area (nothing to do with brake temp). The FDU microprocessor will then filter the data with AND logic and if all channels indicate fire, a message will pass to the FWC and on to the EWD. MLG fire will also call for a speed limit of 250kts (to enable gear to be dropped), Gear down, ATC notify, and LANDASAP.

The only info I have relating to brake temp as a system states that all brake temp info is sent to the BTMU for each braked pair. This amplifies the signal to the BSCU and the info is then displayed on ECAM via the normal routes. If brake fans are fitted (Virgin don't) then the BSCU will also light up HOT on the BRK FAN PB/SW. The brake fans only run on ground with inputs from the switch and the LGCIU in command.