It is is quite easy to find examples of English translations of French patents incorporating your phrase or similar phrases, e.g.:

http://209.85.229.132/search?q=cache:HWMUPnltnxAJ:www.wipo.i...

As you have noted, patent drafters have to come up with all sorts of different phrases to make distinctions between possible features or embodiments and to specify degrees of preference. Some of these may sound strange in everyday English, but are quite common in patents, e.g. if you google "very preferably", most of the hits are patents. Thus, in English-language patents you find phrases like "preferably", "more preferably", most preferably", "very preferably", "particularly preferably", "even more preferably", "in a preferred manner", "according to one preferred embodiment", "according to another preferred embodiment", etc. I think this example includes most of these (and others):

http://209.85.229.132/search?q=cache:MZws993A6SgJ:www.faqs.o...

I would advise you just to stick as closely to the original French as you can. If you google the various English phrases you will find lots of other examples of how they are used in patents.

As for "preferentially", although it is indeed found used this way in patents, this is almost always in patents that have been translated into English - patents originally written in English normally use "preferably" under these circumstances.

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Note added at 1 day21 hrs (2009-07-14 09:40:12 GMT)
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To illustrate the point that there are many different possibilities, one more example for you - an English-language patent including the terms "preferably", "more preferably" and "most preferably", with a typical French translation.
http://www.wipo.int/pctdb/fr/wo.jsp?IA=US1997022715&DISPLAY=...