I have a deviation coming up next month where I am supposed to go MEM-EWR. I am going to deviate from home and go SEA-EWR. Since both legs of the SEA-EWR deviation are scheduled for over 5 hours I booked them in 1st Class (or whatever passes for 1st these days). I have more than enough in my deviation bank to cover this and even leave a little extra for the van ride home. My question is can I legally do this? I didn't see anything in the contract forbidding it, but since I am relatively new and this is my first front end dead-head, and the first month in which I will have a bank...I thought I better try and get some answers. Any help will be greatly appreciated!

Yes, you can. And the +5 hour rule applies regardless of what means of transportation you end up taking. I've done the bullet rain from NRT to KIX a couple of times. The pairing shows a coach class ticket. The train takes over five hours so I booked it in first class.

gcsass, it's explained pretty well in Section 8 of the contract-

Section 8 - Deadheading

4. Class of Service for Commercial Deadhead Travel
Requirements regarding class of service shall apply when the deadhead is scheduled...
a. Deadhead travel shall be booked in coach class unless a higher class of service is authorized.
b. A class of service higher than coach shall be authorized if:
i. a deadhead exceeds 5 scheduled block hours; or
ii. a deadhead is included in a single duty period exceeding 11:30; or
iii. the scheduled block hours of a revenue flight plus the scheduled block hours of a deadhead in a single duty period exceed 8 hours.

C. Deviation From Scheduled Deadhead

3. Deviation Options and Qualifying Expenses
Subject to the limitations and reporting provisions in Section 8.C.4. and C.5. (below), a pilot's air travel, train travel, surface transportation, hotel use, parking and non-taxable per diem are allowable/reimbursable expenses as provided in this paragraph.
a. Air Travel
i. In the following circumstances, air travel expenses are allowable/reimbursable:
ii. Air travel expenses shall be limited to the accepted fare for coach class unless the deviation flight would qualify for a higher class of service under Section 8.A.4.b.

So, yes, you can book in first class when deviating if: a) the deviation flight is scheduled over 5 block hours, b) any two or more flights (any combination of passenger or Fedex operating or jumpseating) in the same duty period add up to over 8 scheduled block hours, or c) your total duty period for any combination of flights mentioned above exceed 11:30 duty hours (don't forget the 1 hour prior show for the first flight).

I've done the first class upgrade (over 5 block hours) MANY times while deviating. Never had a problem getting paid so long as I had enough money in my bank ...

What if your dev bank is fat and you are on a coach flight that is less then your scheduled ticket...at check-in the screen says you can "upgrade" to a seat with more leg room for $X, which when combined with your dev ticket price still keeps you below the cost of the scheduled ticket....can I pay the $X with my FEDEX credit card and then charge that against my dev bank?

It's not an upgrade in class --- just an upgrade to a more comfortable seat.

If allowed, has anyone actually done this...and was filing for this additional expense easy?

Yes, you can upgrade, but you have to pay the difference between the available coach fare that corporate travel booked for you and any upgrade. I've upgraded several times on Alaska from coach to first when the fare difference was only $20-$30. Well worth the money out of pocket.