I think part of the problem with figuring out a way to untangle dialogue from major effects on the game (such as gating content, even if it doesn't affect stats directly) without making it seem hollow and unimportant is because in real life what you say and how you say it matters so much. Even the most honest person in the world will phrase things more favourably. The only way I could really seeing it work is if you had the option to somehow indicate to the game you're lying to get what you want, then maybe some npcs are smart enough to call your bs, or point out inconsistent behavior, and some other times it just works either cause you've got great reputation, the npc is gullible, or you are good at lying and manipulating.
Ultimately though I think this might be a crossroads that can't be bridged: either you punish a player for not roleplaying (which feels like you're being cheated out of part of the game, or like you're doing something wrong, or making the game harder than you want it to be; in any case it doesn't feel good as an entertainment experience), or you don't have the roleplaying have any effect, and it feels hollow.