Cosmetically unsatisfied with length of dental implants. Can bonding or composite be used to lengthen them temporarily? (photo)

The length of your crowns are function of implant position, bone level, soft tissue thickness, shape of the abutment and supporting crown and overall tissue biotype (think vs thick). Your restorative dentist should assess all of these factors carefully and see which is possible to modify to change the gingival margin. If you want to have longer teeth, then it may simply be possible by increasing the contour of the abutment and crown which will result in displacement of gingival margin upward. The reverse often allows movement of gum tissue level downward. This can be tested using provisional abutment and crown until the desired level is achieved.

I recommend that you go to the dentist who placed your implant crowns and express your concerns and dislikes about these crowns. May be you can get them redone.

I agree with you that the laterals could ideally be longer. Composite can be bonded to porcelain but in a high stress area it may not stick well. I suggest talking to your dentist and letting him know that you've always been disappointed with the length of these teeth. Maybe there is a mutually agreeable amount that you could afford to have these redone. In the future, I suggest wearing them temporarily cemented to make sure you are totally happy. 

Obviously, replacing the crowns is the best choice but your dentist could bond composite to lengthen temporarily till you could do the ceramics.  Problem long term is that the composite may separate, stain or come off the crown material hence become less cosmetic in a couple of years and need replacing again.  It would be a great way to determine how long the new future crowns should be and make sure your bite is not traumatic to the new length.