But this is only if they are flying on OW right? That has always been my assumption so I hope this is not a OW Emerald benefit I've been missing out on!

The benefit of OW elites getting into the lounge for a OW flight wouldn't help the OP - he wants to get into a lounge irrespective of airline. The only case where I've heard of this for free is Finnair Platinum (and I think Gold) in Helsinki. I don't know if it is still the case but it used to be possible to get into the lounge irrespective of what airline you were on or what class you were in. I think that KLM Platinum was similar in AMS but I'm not sure of that (never got to Platinum and the benefit may have gone when Flying Blue started)

Back to the original question... Would it be nice to get into the lounge when on an LCC? Yes. Would I pay annual membership for it? Probably not because I don't fly them often enough. A reasonable (US$10-15?) per-use payment may attract me though