Very oldfashioned German.
'trefflich' comes from 'treffen' like hiting a target.
You can say that the person that wishes 'ein gar treffliches Verrichten einstweilen' wishes something like 'may you efforts lead you to what you wish - in the mean time'. I know that is not really the translation you are looking for, but perhaps you can use it as a lead :-)

I don't know! Trefflich from treffen? Maybe very, very early. I have to come to know trefflich as vorzüglich, sehr gut. We need to know more context to find out which other German or English word fits.

Ein gar trefflich(es) Mahl.

I don't know! Trefflich from treffen? Maybe very, very early. I have to come to know trefflich as vorzüglich, sehr gut. We need to know more context to find out which other German or English word fits.

would be a possible translation to modern German?

There is a sense of old-fashioness, and a
connotation of farewell and departure as well,
so maybe, this one is "true and good"...

I lke Thomas version best. At least till now :-)

Have an admirable performance for the time being!

In your translation you should be aware that this is "medieval German" and not spoken today!
You can hear this "Marktsprache" on medieval markets which are very modern in Germany.
There you will meet people in "Gewandung" = clothes from this ages and they try to speak as it was common in medieval times.
Also a lot of old fashioned "Spielleute" = gleeman, musicians, can be seen there singing in this "language". When you look for
"Mittelalter" in google, you will find a lot of pages using this language, possibly as well a page for a better translation to ensure your translation sounds like the original.

If you speak like that today in modern environment it will sound very funny, especially the words "trefflich" and "Verrichten"!

A word that is used as well today is "vortrefflich" and I would say it has the same meaning. In LEO it is translated to "admirable".
The modern equivalent for "Verrichten" could be "Verrichtung", which is "performance" in english.
However, the translation above may be wrong as I translated word by word, and it may also sound wrong.

My dictionary comes up with the following translations for 'trefflich'..splendid, excellent. It can apparently also be used as an adverb as splendidly, excellently.

There is a phrase in English 'Live long and prosper'.
I don't know whether this is entirely appropriate her or not. It could maybe be used for a more long-term farewell than 'for the time being, in the meantime' (einstweilen).

"Live long and prosper" is the "Star Trek"- equivalent to the Jedi-Knighish "May the force be with You" and as such the shibboleth of any Trekkie worth his salt - just a another little
wiseacre addition to the bill ...
It may sound a little old-fashion as well, but
in fact will reach our shores only in the year
2200 from Planet Vulcan - if Mr. Roddenberry didn't get it totally wrong ...

How can you live long in the meantime?

Depends on the temporal extent (or even extension) of that "meantime".