Depending on what low level dungeon we are talking about, i would say.... yes and no.
Yes in the fact that its entirely possible to hold aggro 100% of the time..... IF the mob's life expectancy is low... and your DPS arent just synced down nuclear warheads (lookin' at you, archer, and thaumaturge)
No in the fact of... well, resources arent unlimited. in a bubble, and mathmetically, a tank has the POTENCY of emnity to keep up, but in practice, cooldowns and resources dont line up. an Archer's DoTs and AoE deal far mor Emnity for far longer than a Gladiator can Flash or a Marauder can Overpower. the only low level tank with ACTUAL limitless pool of emnity is DRK, but if we're talking newbie sprouts? well they aint going to be stepping into sastash the first time as a DRK.
I would actually say emnity in the lowest lowest levels (depending on what synced monstrosity is there to carry y'all), is much more akin to a race. its your emnity generation and your TP/MP vs their DPS. the finish line is the boss' HP.
Most of us tanking vets have the synced gear and general know-how to win this race. we all know the "power" of flash and overpower. we know how many we can or cant use so we dont hold up the group. all we CAN really say is "use flash/overpower! alot!" mainly because its the only tool we have synced that low and the only tool that can really win that race most of the time.
As lazy or as simple as it sounds, thats what it comes down to. emnity is an absolute. you either have more emnity or you dont. it doesnt matter if that bar is full. that BLM can be 99 and you can be 100, you're still the target.
All that said, I still believe that with the tools given to us that low, that in a fight that is literally endless, you will lose emnity 100% of the time. LUCKILY.... things will usually die before that happens