Your suggestions are most interesting. As you point out, the projectors have much less resolution than todays dSLRs. If you feed a full resolution image to the projector, it will resample it with possible image degradation. If you know you will be using a 1300 by 720 projector, would it be best to downsize with bicubic sharper before applying your suggested sharpening?
A little off-topic, but relevant I believe.
The answer to that question is yes. Using a good interpolation algorithm will nearly always make the projected image better when downsampling is necessary.
The other issue concerns the type of program (software) used to actually create your slideshow and whether you will have stills in the sense that you are only projecting a fixed image, or whether you will have movement such as pans, zooms, etc.
If your slideshow is very simple with manual advance, etc., there are fewer issues that if you have transitions and especially if you have the Ken Burns effects (Pan, Zoom, Rotate, etc.). There are essentially two types of slideshow programs - ones which have these effects and ones which simply show sequential slides. Simple software with slideshow capabilities such as IrfanView can be used to produce beautiful stills but if you want Pan, Zoom, Rotate and transitions you need a serious slideshow program capable of hardware rendering at the resolution of your projector.
Currently the only software which does hardware rendering is PicturesToExe, but apparently ProShow (Gold and Producer) are working to get hardware rendering so it may be coming from several different providers in the future.
When you prepare images for projection, as mentioned earlier you need to interpolate with a good algorithm (bicubic is fine) to the projected resoltion. If there will be zooms, pans and rotates, you may need to take a bit of the edge off sharpening. If the image will be displayed with no movement this is of less importance, but movement, especially zooms, requires that the program resize on the fly and this tends to cause "sparkle" and some pattern artifacts which are exacerbated by overly sharp images. Consider this when preparing your slides (images).
Some of the new software such as PicturesToExe (at Beta 5 of about 7 now) has incredible power to produce everything from simple still shows to elaborate movement and animation. It's a new world folks when an inexpensive software program can do things which formerly took studio technicians months to achieve.