I suggest you contact Ermin Elezovic. His contact number is in Rick's Croatia/Slovenia book (in the Mostar chapter). EDIT: Here is his email address: firstname.lastname@example.org. Ermin has a very comfortable car, and drives around the area (including into Croatia). Ermin's wife (forgot her name, but I met her in Mostar - wonderful!!) does guided day trips in Mostar and the area - she is also described and recommended in RS's Croatia book. They also have a couple of other drivers in their business who might be the ones to pick you up in Split and get you to Mostar.) Between Ermin and his wife, if either is available, you might find that they can help you arrange a fabulous couple of days. I am pasting in my description of my day with Ermin in June, 2018:
This was the most beautiful, insightful and enjoyable day of my trip. I pre-arranged with Ermin Elezovic (in the RS book Croatia/Slovenia), p. 429) to pick me up at Panzion Cardak in Mostar and eventually deposit me at the Old Town Hotel in Sarajevo. Ermin is warm, witty and wise. He was dependable, helpful, a great driver, and loves his country passionately yet with insight and honesty. Immediately in the morning we went to the historic gorgeous Sufi site at Blagaj, where the old Sufi Dervish house is built right beside/over a river and beside the cave that is the source of the river. Ermin got us there before any other tour groups or individuals, so we wandered through the house alone, feeling the strength of the ancient stones, the water, the soaring swifts overhead, and the cliffs hanging high above us. He knew about each room, its history and use, and clearly loves and reveres the place. Next we visited the historic old village of Pocitelj, where I explored up high stone stepped streets, and then rejoined Ermin to relax at the shady table of an outdoor cafe run by a friend of his, enjoying the views and the cool drinks. Finally, we had an incredible lunch at a country restaurant down a quiet lane, alongside a small river, that had pools full of fresh trout, and a stone mill where they grind their own polenta. Wowee! For under $10 US I had fabulous grilled fresh trout (those guys were probably swimming earlier that morning), polenta with herbs and butter, salad and bread. Oh my. And we sat in the shade beside a pool, watching other trout swim unaware of their eventual grilly fate. Finally, Ermin drove me into Sarajevo and made sure that I was within a few steps of my hotel. This private tour was my "splurge" of the trip and worth every penny (Bosnian mark).