There are a number of reasonably priced Dual WAN routers that support
load balancing and auto-fallover. Search the electronics section of
amazon.com for "dual wan" to get some model numbers.

Hi Snowbat, thanks for the reply, but this is not appliable for a number
of reasons:
- the situation is a little more tricky. The main router is a L3 switch
stack placed in the local headquarter and reachable from the subsidiary
warehouse (which is on a separate Vlan/subnet) with a WiFi bridge, while
the backup router is on-site in the warehouse.
The failover is in case of a wifi breakdown (this is the reason I need
to ping the router and not the web).
The web connection is given through wan connection to a proxy and a gw
in our datacenter in Vienna (I'm speaking about a _very_ big company,
the one that produces that very famous brown, sparkling, not-alchoolic
and sweet beverage...).
And finally the connection isn't a simple ADSL but an SHDLS which
require different HW and support for MPLS wan.
What you proposed would be good if it was an auto-failover lan router
with 3 nics.

Check out the "connectivity check" options on the ZyWALL 70.
http://www.zyxel.com/guidemo/zw70_demo_gui_b2/index.html
It can ping the default gateway or ping any address you specify.

Frankly though, I find your boss' objection baffling. Why buy an
expensive, overly-specialized tool when a totally generic one will do
the job just as well?