gab,
First just so you are aware. This site MyOpenRouter.com is not owned or affiliated with NETGEAR®. So you may not get an official answer here regarding warranties but may get something more like opinions.

And here is my opinion. :)
First of all, any issue I have gone to Netgear with for warranty has always been taken care of.

But their warranty specifically says
"NETGEAR products are covered by a limited liability warranty from defects in material and workmanship. This warranty does not apply if, in the judgement of NETGEAR, the product fails due to damage from shipment, handling, storage, accident, abuse or misuse, or if it has been used or maintained in a manner not conforming to product's instructions, has been modified in any way, or has a defaced or removed serial number. "

Which I can totally understand. Say for instance you installed DD-WRT or Tomato and cranked up the wireless transmit power to way above specs and burned it out? 

That would not be a fault of the firmware or the hardware but could be considered a user abuse type situation.  So its not simply a matter that you installed a 3rd party firmware but what you did with it.

So there can be some grey areas. I have always felt that once I add a 3rd party app to a router that I can't expect the manufacturer to warranty something out of their control.

But like I said I have always had great support from Netgear on any product I have purchased from them.

In the end if you are worried about warranty then I would say stick to the stock firmware. Which from your original question of how you are going to use it should work fine.

But again these are just my opnions.

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