Two random interesting bits thrown up by my current research:
Nandi, a Nilotic language of Kenya with VSO order, would appear to allow you to relativize virtually any constituent of a sentence. I was particularly impressed by examples like:
nikò ce:pyó:sé:t ne â:-nken ci:tà ne kí:-ká:ci kitâ:pú:t
this woman Rel 1s-know person Rel Past-give book
"This is the woman that I know the person that gave [her] a book / that [she] gave a book to."
á-ké:r-é ci:tà ne pè:nt-í: àk la:kwe:-nyi: kâpsá:pit
1sg-see-impfv person Rel 3pl-go and child-his Kapsabet
"I see the person who [he] and his son are going to Kapsabet."
Take that, Subjacency Constraints! (Well, more seriously, I'm guessing ce/ne is probably not a fronted relative pronoun, especially since it agrees in case with the head noun and not with the position of the gap, so maybe no movement is involved - but that just raises other issues, like what does the gap consist of? Surely not pro? And what is ce/ne - a complementizer?)
And, in case you've ever wondered what an Arabic incomprehensibly double center-embedded sentence would look like, here's one:
رأى الولد الذي كتب الرجل الذي عينه الرئيس الرسالة إليه أخاه
ra'aa lwaladu lladhii kataba arrajulu lladhii `ayyanahu rra'iisu rrisaalata 'ilayhi 'axaahu
saw [the-boy [that wrote [the-man [that chose-him the president]] the letter to him]] brother-his.
“The boy the man the president chose wrote to saw his brother.”
Note that Arabic's VSO order renders it less vulnerable to subject- and object-relativization in this regard, but leaves it helpless against relativization of other positions - which is nonetheless permissible.
(Nandi examples from Creider & Creider 1989.)