I think that even though "Klobürste" is marked "ugs." in Duden, and "toilet" is obviously not actually slang, I still support the suggestion, not purely because of how often they are used but because "Klobürste" can be used in more (and more polite) situations than "bog brush", which is really pretty coarse. Klobürste reaches further towards the polite than "bog brush", and "toilet brush" reaches further toward the informal than "Toilettenbürste" - I think they overlap in the middle.
Langenscheidt Muret-Sanders has Klobürste: loo (AmE toilet) brush and Collins/Langenscheidt e-GWB has Klo|bürs|te f (inf) toilet or loo (Brit inf) brush .
(but actually I wouldn't say loo brush myself, and it doesn't get all that many google hits)
"bog" is more coarse:
bog 2 coarse Brit, slang a toilet. http://www.chambersharrap.co.uk/chambers/feat...
The above joke shows that "toilet brush" is not limited to polite conversation.
Klobürste gets quite a lot of hits from this quite formal newspaper:
33 from zeit.de for klobürste (11 from zeit.de for toilettenbürste.)
Ängstlichen Angestellten wird gern empfohlen, sich ihren Chef auf der Toilette vorzustellen." http://www.zeit.de/2003/47/Luxus_2fAlltag_2fK...
...even used in connection with a priest:
"Folge 1: Der Kampf mit der Klobürste Folge 1 Veit Hoffmann; Pfarrerssohn; ist heute Pastor in Berlin" http://www.zeit.de/archiv/2000/21/200021.erin...
There are not as many newspaper examples of "bog brush":
Results 1 - 4 of about 8 from guardian.co.uk for "bog brush".
(All of the Guardian quotes are humorous comments about unattractive hairstyles)
3 from timesonline.co.uk for "bog brush".
"I can only assume that, even as he was chucking the last bog brush in the back of the removal van and wiping away a tear for 15 happy years at number 46,"
"Toilet brush" comes up more often, and not only in formal contexts:
38 from guardian.co.uk for "toilet brush".
"Prescott would be a toilet brush.... Ming Campbell the spanish lady in the frilly dress covering the loo roll" http://commentisfree.guardian.co.uk/peter_fra...
13 from timesonline.co.uk for "toilet brush".
"the Merdolino toilet brush has to go, as does the kitchen-roll holder featuring a bunny and a carrot." http://property.timesonline.co.uk/article/0,,...