I can't think of anything inherent to a freewheel design that couldn't allow the lowest cog to sit as close to the spokes as a cassette/freehub would. A little relief in the rear of the body and a little offset to the cog would get it there. That's in fact what the Suntour Winner Pro freewheels had. The limit isn't the design of the cog carrier, it's the rear derailleur clearance.

Also, most freewheels that I'm aware of made since the 1970s have had splined sprockets like cassettes, or at least the innermost four or five are splined.

I think the reason freehubs got cogs closer to the spokes is simply because they had to as the number of rear cogs continued to increase past freewheels' practical limits. Freewheel development pretty much stopped cold after about 1990 because of axle overhang and breakage, and the increasing dominance of the Shimano freehub design. Shimano's integrated design also eliminated dimensional variables that might cause clearance issues when mixing hubs and freewheels from different manufacturers.