Well, this sure ain't pretty but maybe someone will improve on it. Not me. To me this is just that tiny bit better than just using misc and I don't need more. This is based on biblatex-juradiss and I kept the German comments. The fields are explained in the German biblatex-juradiss documentation. Well, this is optimised for German court decisions anyway so maybe this is not too catastrophic.
\DeclareBibliographyDriver{jurisdiction}{%
\usebibmacro{bibindex}%
\usebibmacro{begentry}%
\newunit\newblock
\iffieldundef{usera}%Urteil oder Beschluss
{\nopunct}
{\printfield[default]{usera}\space%
of\space{}the\space}%
\printnames[default]{author}%
\space{}from\space\usebibmacro{date}%
\newunit\newblock
\printfield[default]{userb}% Aktenzeichen
\iffieldundef{userc}%Primaere Fundstelle (Name und Jahr) vorhanden?
{\nopunct}
{\space%
\printfield[default]{userc}}%
\iffieldundef{postnote}{%Angabe einer konkreten Fundstelle?
\iffieldundef{userc}{}{%
\addcomma\space
\printfield[default]{userd}}% Erste Seite der primären Fundstelle
\iffieldundef{journaltitle}{}{%Sekundäre Fundstelle vorhanden?
\addspace\mkbibparens{%
=\space%
\printfield{journaltitle}\space%Zeitschriftentitel
\printfield{usere}\addcomma\space%Zeitschriftenjahrgang
\printfield{pages}}%Erste Seite der Zeitschriftenfundstelle
}%
}%
{}%
\iffieldundef{userf}{}{%Entscheidungsname vorhanden?
\addspace\printfield[parens]{userf}}%
\newunit\newblock
\usebibmacro{url+urldate}
\usebibmacro{addendum+pubstate}%
\setunit{\bibpagerefpunct}\newblock
\usebibmacro{pageref}%
\newunit\newblock
\usebibmacro{related}%
\usebibmacro{finentry}%
}
According to the documentation you need this in a biber.conf file that is put next to your document. For me, the file is read by biber but it does exactly nothing at all.
<map>
<bibtex>
<globalfield>
decision usera
sign userb
officialvolume userc
officialpages userd
journalyear usere
decisionname userf
</globalfield>
</bibtex>
</map>
Thus for the example in my question I used this entry where all user* fields are present twice, hoping that maybe one day I can remove the cryptic ones.
@JURISDICTION{bvg1983,
author = {{Federal Constitutional Court of Germany (BVerG)}},
decision = {Judgement},
usera = {Judgement},
sign = {Az. 1 BvR 209/83, 1 BvR 440/83, 1 BvR 420/83, 1 BvR 362/83, 1 BvR 269/83, 1 BvR 484/83},
userb = {Az. 1 BvR 209/83, 1 BvR 440/83, 1 BvR 420/83, 1 BvR 362/83, 1 BvR 269/83, 1 BvR 484/83},
owner = {cmertes},
urldate = {2012-10-11},
language = {german},
url = {http://openjur.de/u/268440.html},
urldate = {2012-10-11},
date = {1983-12-15},
decisionname = {Volkszählungsurteil},
userf = {Volkszählungsurteil},
timestamp = {2012.10.11}
}
This produces the following result. For everything else, I take no guarantee. But please correct any mistakes I made because BibLaTeX is very new to me.

biblatex-juraorbiblatex-juradiss? The latter comes with a German documentation and exactly your wanted entry type. – Speravir Oct 11 '12 at 00:43jurisdictionas a synonym ofmiscand stop caring. Writing (or copying) a whole legal citation style for one entry is really not worth it (both in terms of time and in terms of code / memory). – ienissei Oct 11 '12 at 06:45biblatex-juradisspackage that @Speravir suggested and tried to change it in a straightforward way to a bibliography style (which the package doesn't provide). It was worth it in the sense that I learned that adding "Urteil" is absolutely necessary for the thing to make sense (I had just left it out). If the result looks ok I will post it here even though it will be a hack. If not, I will go for Plan B ;) – Christian Oct 11 '12 at 07:03