AVR-Chat

I preface this question with the fact that I have very limited knowledge of
CAN bus and DeviceNet technologies (I do have a background in data
communications from both the EE and software engineering perspective).  I
have only 'skimmed' the description of CAN bus and DeviceNet on Wikipedia.
I know that there are Atmel devices such as the ATMega16M1 that implement
much of the CAN network support in hardware (skimmed the ATMega16M1
datasheet).

I understand that CAN works well for 'local area' sensor networks such as in
a automobiles, etc. but can I leverage the Atmel CAN support for a network
that is physically much larger (i.e.: one with significant propagation
delays)?

I am currently implementing a network of (hopefully) low cost network nodes
using ATMega16s connected via multi-master, half-duplex RS-485.  The network
is:

1) Outdoors
2) Length: 1,000s of feet using one pair from a CAT-5 cable (other pairs
used for other control tasks).  I anticipate breaking the net into subnets
with intelligent (buffered) 'bridge' devices if the physical layer requires
it.
3) Bus/power surge protection provided by transorbs, etc.
4) Data rate: As fast as practical to reduce message latency but I don't
anticipate needing an excessively high data rate.  For the moment let's say
on the order of 100 kbs.
5) Message Traffic: Most messages are broadcast type messages containing
status info that any node may consume to support its work, others are
addressed to specific nodes.  Some messages are more time sensitive than
others but there are also messages that are sent more or less continuously
as bus bandwidth allows.

Would it be possible/practical to use the Atmel CAN support in this
environment?

I read somewhere that CAN was used up to 400m due to timing considerations.
Can this distance be extended by altering (possibly 'non-standard') timing
parameters within the AVR hardware?