Beamer: Spacing in top-aligned frames and non-frame pages

Usually, all pages in beamer documents are generated by frames, but beamer also allows documents without frames or mixed documents. Any content that is not part of a frame is printed nearly as if it would be on a top-aligned frame with the allowframebreaks option. This is especially useful for documents generated from a source that was not meant to be used as presentation. Obviously these "non-frames" do not support overlays, but if one just needs overlays on certain slides, this is not a problem.

However, these "non-frames" do not behave exactly like frames, e.g., the gap between the headline (if any) and the frame content is wider for top-aligned real frames:

documentclass{beamer}

% makeatletter
%   define@key{beamerframe}{t}[true]{% top
%     beamer@frametopskip=-0.9mmrelax%
%     beamer@framebottomskip=0pt plus 1fillrelax%
%     beamer@frametopskipautobreak=beamer@frametopskiprelax%
%     beamer@framebottomskipautobreak=beamer@framebottomskiprelax%
%   }
% makeatother

begin{document}
x
begin{frame}[t]
  x
end{frame}
end{document}

The example above creates two pages containing just one x, but the x on the second (the real frame) one is positioned lower.
Looking at the beamer internals, this can be circumvented by using a negative beamer@frametopskip for top-aligned frames (commented out).

This has two drawbacks:

• Internals may change and thus this code might break in the future.
• The chosen value for beamer@frametopskip is a rough guess and likely depends on certain lengths, skips or the theme.

Thus my question:
Is there a robust method to make non-frames and top-aligned frames behave as similar as possible, especially w.r.t. the top margin (i.e., headline-content distance)?

If there is none, it would be at least helpful to get the optimal value for beamer@frametopskip to undo every skip etc. inserted on regular frames.

PS: A solution that handles "non-frames" like regular frames is also fine, I do not need minimal margins, just consistence.