cc65/ProDOSi/Text Adventure file buffer issue

Hi!  I have cc65 and am working on ProDOSi and thinking about porting some text adventure skeleton code to the Apple2.  I want to have the last be a system program and use the space between $0800 and $2000 for extra code and data and I/O buffers.  I only need two (or maybe one) files to be open at a time.  How do I use ProDOS to set the buffers?  ProDOSi so far can open and close files, and I'm currently working on read/write access.