I go to Ghana once per year and am always casting about for a decent rental bike. I've almost always ended up with a small, ill-maintained local bike, but I'd definitely pay for something larger which had been maintained to Western standards.
There's a market, but it's been historically small. The last regular rental business was at the Arts Center, and I don't think it ever really took off. When the American/Ghanaian couple which owned it split up (I gather), the business folded. There's a German national with a house in Kokrobite who claims to rent big bikes, but my experience with his company was less than satisfactory.
One more useful point: Ecowas insurance would be a real plus, since that would allow cross-border rentals and therefore longer trips. This could be an optional extra.
Post back here if you're proceeding. I'll be back in West Africa late this year or early next.