A comment to another question of mine asked me "Do you know any separable loan word?".
I can't think of any, off-hand. Are there any? One would be enough to answer the question.
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As you can infer from the other answer and comments below, we would have to narrow down the definition of a loan verb.
If it were sufficient enough that the root of the verb is not of Germanic roots, then there are certainly verbs.
All these seperable verbs, however, have a Germanic prefix. These are not pure non-Germanic verbs and to my mind we should rather take a look at verbs that have foreign prefixes. Pure non-Germanic words.
Having taken a look at the list of foreign prefixes on canoonet, I can't think of any verb with those prefixes that is separable. I dare say there is no such a verb.
EDIT: This thread is rather old however after living in Germany for some time I would like to add a couple of comments. This semester I'm also taking a course "Lexikologie des Deutschen" which will be dealing with neologisms in detail, so there may be more to come.
Specifically I would like to mention that English phrasal verbs that use the adverbial particle "up", or even those verbs that are prefixed with "up" (as in "upgrade", "update", "fuck up", "upload" as specific examples) have a special affinity for being treated as separable verbs in German.
This is ostensibly due to the fact that "up", when rendered with German phonemes (phonetically [ap]), is pronounced exactly like "ab" (also [ap]), and provides some kind of "pressure" on the speaker, at least when conjugating the past participle, to say "upge-" (like "abge-") and not "geup-" (like "geap-"), the latter of which would usually be an illegal in native separable verbs. Given that the English verbs "upgrade", "update", and "upload" are not phrasal verbs, they are far less often separated in inflected forms (as above from @Mawg says reinstate Monica: "Hey, du, date das up!" -> rather unlikely).
Despite this, I have often heard "graden ... up" and similar separated forms of the other words I gave as examples from native speakers, usually accompanied by comments along the lines of "hmm, oder upgraden? Egal!" - and then the other way around - da gehen die Meinungen wohl auseinander!
On the other hand, the loanwords that are verbs in English that are phrasal (at the moment only "to fuck up" comes to mind) are always conjugated separably. Not only as an adjective from the past participle (abgefuckt) but also as an inflected verb ("Das fuckt mich richtig ab, alter", as a man at Frankfurter Hauptbahnhof who had not yet received his Hartz IV for the month once told me).
It should be noted, though, that the popularity of "abfucken", according to Wiktionary, has been pushed by the word "abgefahren", at least adjectivally, due to the related notion deriving from "crazy, out of this world, gone by the wayside" -> "fucked up".
Hopefully there will be more interesting information to come in the coming weeks here.
"Downloaden" and "uploaden" are mentioned by Wikipedia as sometimes being treated as separable verbs, depending on the speaker (i.e. "down/loaden", "up/loaden").
Something of interest is that there are plenty of verbs that have been borrowed from English that, whilst don't have a separable prefix, have an inseparable one whose stress doesn't lie on the first syllable and thus does not take "ge-" in the past participle.
Examples off the top of my head include: