Don't know for Split exactly, because I'm not from there, but it shouldn't be a problem to book a small bus, especially for someone whose job is a tourist guide. :/ I think he is not willing to do that in fact.
You can try to rent a bus by your own.
Here is one contact in Split: http://www.lavandatours.hr/en/
The price as I know is about 2000 kuna for one day trip.
If I'm from Split I will do it for you, as I'm a mountaineer guide and do this stuff (booking bus etc).
If you come one day somewhere near Rijeka, me and my boyfriend, also geocacher and mountaineer guide, will gladly took you to some peaks with caches. :)
