Different editor for collection and maintitle

Different editor for collection and maintitle

BibLaTeX/biber seems to be unable to handle the following case: A
@Collection is part of another series-like collection (series has already
been used), for which I use maintitle (series has already been used). But:
The @Collection's editor is not the maintitle's editor ...
To give an example how it should look like
@BookInBook{Erler:PlatonHaBu,
title = {Platon},
year = 2007,
author = {Michael Erler},
gender = {sm},
booktitle ={Die Philosophie der Antike},
editor = {Hellmut Flashar},
maintitle ={Grundriss der Geschichte der Philosophie},
language = {german},
volume = 2,
part = 2,
publisher = {Schwabe},
location = {Basel}
}
becomes (using BibLaTeX/biber with style=philosophy-modern, which
shouldn't make any difference)
Erler, Michael
2007 Platon, in Grundriss der Geschichte der Philosophie, Bd. 2.2: Die
Philosophie der Antike, hrsg. von Hellmut Flashar, Basel: Schwabe.
Excellent: 20 volumes Outlines of the History of Philosophy (maintitle)
--> part of it: Ancient Philosophy (booktitle, editor is Flashar) -->
single volume: Plato (title, author ist Erler)
BUT:
@Collection{JongNuenlist:Time,
editor = {Irene J. F. de Jong AND Ren{\'e} N{\"u}nlist},
sortname = {Jong, Irene J. F. de AND Rene{\'e} N{\"u}nlist},
gender = {pp},
title = {Time in Ancient Greek Literature},
booktitle = {Time in Ancient Greek Literature},
shorttitle = {Time},
maintitle = {Studies in Ancient Greek Narrative},
volume = 2,
language = {english},
year = 2007,
series = {Mnemosyne. Supplementum},
number = 291,
publisher = {Brill},
location = {Leiden AND Boston}
}
becomes
De Jong, Irene J. F. und René Nünlist
2007a Hrsg., Studies in Ancient Greek Narrative, Bd. 2: Time in Ancient
Greek Literature, Mnemosyne. Supplementum 291, Leiden und Boston: Brill.
I need it as @Collection, because I cite several @InCollections. Neither
are de Jong and Nünlist the editors of the maintitle, whereof there is no
editor at all, but of this @Collection, nor do I want to print the
maintitle before the title of the @Collection.
It should look like
De Jong, Irene J. F. und René Nünlist
2007a Hrsg., Time in Ancient Greek Literature, in Studies in Ancient Greek
Narrative, Bd. 2, Mnemosyne. Supplementum 291, Leiden und Boston: Brill.
The latter I get with booktitleaddon, which is un-TeXy, messy and unordered.