Sorry, this is rather long.
The only rules where I have seen points for engineers is in Elan Delux. It would seem that engineers are the same cost as infantry of same grade (line, veteran etc). A good engineering officer should be a rare thing and should cost more than this I would think. When taking into account the sappers and miners, the cost of 30 points for a company seems appropriate.
I have not yet played a campaign, but have given some thought to some of the factors which may affect a campaign. There is some good information in Charles Grants "Napoleonic Wargaming".
I think using points initially is OK, but replacement of some arms was more difficult than others. A limit on the ability to replace artillery and engineers would be appropriate, along with Guard units, cavalry in general, and cuirassiers in particular. This will prevent forces being thrown away knowing they can easily be replaced. It might also lead to (Napoleons) unwillingness to commit the Guard.
Then, many of the losses in the game represent not just casualties, but the degradation of the forces ability to fight during that particular battle. Reading Nosworthy and Rothenburg's books will give some idea as to the kill:wound ratio. A surprisingly high number of wounded returned to active service (sorry, I don't have the books in front of me to give figures).
Forced marches will increase attrition. Guard units may suffer less attrition due to better medical care and provisions, as well as being used to the rigours of campaign. Conscripts will melt away on forced marches.
If lines of communication are cut you may have problems with provisions and replacements.
Captured rear areas may mean your wounded are captured and cannot return.
One of our local gamers is keen on the idea of rewarding units that do well in a battle with an increase in grade, while those that have done poorly lose a grade.
Obviously, you can make things as complicated or as simple as you wish.
To reflect attrition, a simple way may be to use the Angus Wargames Campaign Loss Generator.
http://www.geocities.com/anguswargamesclub/awclinks.html
Hope this is of some help (other than as just a sedative)
Ants