Not sure if you're still researching this or not but here are my thoughts:

We did St Lucia to Grenada a few years ago with Horizon. I think they ditched their St. Lucia base - if they even had one (I can't recall). However, they may still deliver a boat to St. Lucia for a one-way return to Grenada as they did for us. Of course you can start at their St. Vincent base and do a one-way to Grenada which I would recommend unless you are on the boat for 2 weeks or more. There is a lot to see between St. Vincent and Grenada, plus starting in St. Vincent will save you from having to clear out of St. Lucia and into St. Vincent.

As I'm sure you know, DYC doesn't have base in St. Lucia or St. Vincent, but they do have a base on the southern end of Martinique roughly 25 miles north of St. Lucia. If you really want to see St. Lucia you could start there and do a one-way to Grenada and it would probably only add one day to your charter versus starting in St. Lucia. DYC is very amenable to one-way charters as long as they have a base at your starting and ending point. That said, it would add to C&I requirements as you'd have to check out of Martinique, into and out of St. Lucia, into and out of St. Vincent/Grenadines, and finally into Grenada.

I believe Moorings has a base in St. Lucia and Grenada - have you talked with them yet?

Finally, since you are hiring a captain you could see if he could grab the boat from Grenada and power up to St. Lucia to pick you up there. Once again, would add a day or so to the cost of the charter, but if the charter company facilitates the one-way you will have to pay a one-way fee anyway. If you add a day and then have the captain meet you at St. Lucia it probably won't be much if any more than the one-way fee. FWIW, we just booked a one-way from Antigua to St. Martin for June and our one-way fee is ~$1,300 - about the same as the daily charter rate for the boat. I was tempted to just add a day and return the boat myself, but it's a long way to go in one day and there is a good chance we would be going straight into the wind/swell. I think the one-way fee was a good decision in this case.

Feel free to PM me if you have any other questions I can help you with!