.. I assume they are not stick built so would the liner restrict access to a lot of areas behind and under settees, counters, etc? I've never been on a new boat to poke around and check that out. If anything, we have too much storage space (really).
One of the things that differentiate these boats from big production boats is the care with detail. These boats are normally boats built for many years by the same guy, that is most of the time also the designer and have an almost familiar relation with their clients. Many of them have already bought several boats along the years and they contribute with ideas about design interior and has the average buyer is not a marina sailor, but a real cruiser, those ideas are many times very good ideas, that are incorporated into the design.
When you buy one of those boats you are not only experiencing the knowledge of the guy that design the boat, but also the accumulated experience of many cruisers that have contributed to improvements on the boat.
Answering your question, yes, you can count that any inch of available space on those boats have a use. It could not be the use you would give it, but then the builder (that is also the designer) would be very interested in hear your ideas and into incorporated them in your boat. And he would be interested because he would recognize in you an experienced cruiser and know than its boats has been improved along the years with the contributions of many experienced cruisers.
I don't know if you are following this thread from the beginning, if not I think that you would be interested into looking to the first posts about the Sirius, also DS sailing boats. In what regards the use of each inch of space they are very hard to beat
Take a look:
Sirius 35 DS - YACHT: YACHT tv
And they are also among the ones that incorporate more ideas from their clients...actually their new 43ft is being designed with the active colaboration of their clients and future clients
Sirius-Werft Plön | 35 DS | Philosophy
For the ones that already know the Sirius 35, their 360º pictures are already on line:
Sirius-Werft Plön | Cockpit/Saloon | 35 DS Cockpit/Saloon
Sirius-Werft Plön | Saloon | 35 DS Saloon
Sirius-Werft Plön | Owner´s cabin/Head/Shower | 35 DS Owner´s cabin/Head/Shower
Sirius-Werft Plön | Forecabin/Head | 35 DS Forecabin/Head
Sirius-Werft Plön | Technic Room | 35 DS Technic Room
And some interesting sailing tests:
This one is in German, but has it is from my favorite boat magazine I will post it (and in the next link a translation):