Certing engies for support can get plenty of certs. Repairs and ammo are key in most battles (alongside medics) and you can do both. Actually, you can do a bit of everything, which makes it a rewarding class to dump certs into. Fly, drive, repair, AT, snipe, fire support and CQB, there's an option for it with an engy, but you'll have to cert into it.
The jack of all trades and the master of quite a few.
Plus, you can just play like a heavy when you feel the need, except things like ammunition discipline don't apply to you. Even SMGs are perfecty acceptable fire-support weapons with an ammo pack, and shotties are far more forgiving when base run'n'gunning (just drop your regular ammo pack and your "b" one at likely looking places and you can solo pretty well with a shotgun). You're debatably better at medium/long range than a heavy, due to bullet hosing and BR use (with maxs being better at close than heavies anyway), with your own weird unlimited ammo AT weapon should you want that role. Admittedly the AT MANA can't do everything or do it quickly, but it's there as an option.
Heavies need medics and engies. MAXs need both as well. Engies just need someone, anyone, doing absolutely anything to earn a few certs. Or a medic. Preferably one with a UGB launcher.
edit: Also, for a support only role, a shotgun is a pretty viable weapon. Think of it more as AI/AA flak. While you're repairing vehicles, ammoing people, etc, fire a few rounds off in the general direction of the enemy. Chances are you'll plink them a couple of times, making it all the easier for your heavies/maxs to take them out, and getting you a hassle free assist. Even with terrible bullet velocity, you can often plink aircraft if you lead them a bit. The spread might be huge and ineffective damage-wise, but it's still an assist (esfs are pretty big targets when all's said and done). Sometimes it even freaks pilots out a bit. You've also got a perfectly viable CQB weapon in your hands to take out whoever took down your MAX/heavy. With its only problem being ammo (doesn't apply to you) and range (you're plinking for assists, not big damage), shotties work well for pure support engies. Great vs LAs or infies that are trying to C4 or AT mine your sundy too. Think flak. Fire indescriminately. Shotties are great.
edit 2: Oh, and kill stealing. Up to about 30m, it might not be you plinking the assist with your shotgun. Sometimes quite a bit further as well. Your heavy either has his ammo or he doesn't, your MAX will tend to live or die in any short-term action regardless of your repair gun. When there's action on, pull out the shotty and use them as body shields. You'll get the kill, the assist, or kill the SoBs that killed your assault troops. Think of all the repairing/ammoing time you saved by just killing people. Works in all situations, with all class makeups. Repairs and ammo happen before and after engagements, at all other times there's shotgun plinking/kill-stealing. Watch your certs jump through the roof.