I don't believe it will be safe but the only reliable source for an answer would be your carseat manufacturer. Take a look in your installation manual and give them a call.
My concern is that the carseat won't be sitting flush against the bottom of the car's seat and therefore won't be a rock-solid install. I'm anal when it comes to installing carseats and if it's not rock-solid, I'm not comfortable having my child sit in it.
We drive down to Disney every year from NJ and have found that having a small pillow to lay their head against or a neck pillow to keep their head from slouching is enough for them to nap comfortably. I also put a crate or box or some other form of luggage in front of their seat for them to rest their feet on. I've found that having some leg support adds to their comfort level - having your feet dangling for a long period of time can cause tingling or discomfort so give them a footrest of some sort.
Good luck and safe travels!
ETA from carseat.org (
http://www.carseat.org/Technical/tech_update.htm#angleFF )
(Bolding is mine, CR stands for "child restraint", certified means certified by the carseat's manufacturer to be safe in that position)
Angle of recline, forward-facing
The most common recommended orientation for a forward-facing CR is the most upright position. Many convertibles and some child seats have two or more choices of seating angle for forward-facing use, and each of these positions must be certified to FMVSS 213. Although the most upright position probably still provides the most effective overall restraint in terms of load distribution to appropriate bony structures, a
certified reclined position may be used for a sleeping child or to make the CR match the angle of the vehicle seat for a tighter installation. It is a good idea, however, to try the most upright position first, because the upright geometry of the CR coupled with the angle of the vehicle seat cushion may sufficiently recline the child to be comfortable.
For children seated directly on the vehicle seat, in a backless booster, or in a booster with adjustable back, the normal seatback angle is about 20° to 30° from vertical. This comfort-angle range also applies to adults. If the seatback is tipped farther back than this, there is a danger of the occupant sliding under the safety belt, called submarining, during a crash. This is not a problem in a forward-facing CR, because the entire shell is reclined, not just the seatback, which reduces the likelihood of submarining. Submarining in a CR is also limited by the crotch strap holding the lap straps down on the thighs.
Potential injury from spinal loading, predicted in the early days of CR design, has not been shown to be a problem in actual crashes. One laboratory study in England found that a forward-facing CR had to be tipped more than 50° from vertical to generate harmful loading to the dummy, and this is more reclined than acceptable for forward- or rear-facing CR use. (9/07)
