In pdftex "if the \pdfpageattr value contains the string /MediaBox,
then pdfTEX omits outputting its own /MediaBox value". That means
that the following produce a PDF with only one /MediaBox in the page
dictionary:

\pdfcompresslevel0
\pdfobjcompresslevel0
\pdfpageattr{/MediaBox [0 0 200 300]}
abc
\bye

The same is not true for luatex:

\pdfvariable compresslevel = 0
\pdfvariable objcompresslevel = 0
\pdfvariable pageattr{/MediaBox [0 0 200 300]}
abc

\bye

leads to two entries (using pdf.setpageattributes gives the same):

/MediaBox [ 0 0 595.276 841.89 ] /MediaBox [0 0 200 300]

Would it be possible to copy the pdftex behaviour also in luatex?