It really depends what you're going for....how much fruitiness do you want in the end product? How much herbal character do you want?
I find that most fruit beers in general tend to turn out dry and acidic with a touch of the fruit aroma/flavor. Lindemans fruit lambics, for example, are pasteurized before adding the fruit and back-sweetened, if I remember correctly, to achieve their sweet fruitiness.
In mead or wine-making, however, you get more of the fruit profile - I'm guessing due to the high volume of fruit and sugar.
So, if you wanted to make a fruit gruit, I would either go really high gravity with a generous amount of honey to achieve a sweeter, fruitier brew, or go into the direction of a lambic: dry, sour, remnants of fruit, and not so high in alcohol.
You have to pick a base herb with high antimicrobial properties such as yarrow, mugwort, wormwood, or sage (I wouldn't personally choose this as a base herb). Yarrow is the most neutral in flavor although it does make brews quite tart and dry. As a tea, yarrow is bitter and has a medicinal kind of floral note. Yarrow beers never turn out with much of these characteristics - always tart in my experience. The bitterness is not really noticeable, but there is balance.
The artemesias (mugwort and wormwood) add highly noticeable bitters and a very herbal flavor behind - more true to their raw aroma and flavor. Wormwood is mainly a bitter and doesn't have all that much flavor/aroma of its own (unless you use flowers). Used sparingly, it could make a neutral base for fruit beer, but I think the fruit would have to be something very sweet and potent (raspberries are a good choice) for balance. Mugwort is not nearly as bitter as wormwood, and you will get more of its character in the end product. This might compliment fruits in the right proportions.
I have actually made one fruit gruit. Needless to say, the results taught me a lot.....
used a belgian golden ale as the base with yarrow as base herb + german and roman chamomile for flavor (mashed low). I then fermented with a combo of plums and pluots (2lbs total per gallon). It came out very dry and sour. All the flavors pretty much gelled together, can't really tell the yarrow from the plum from the chamomile. I ended up adding a sour mix (bretts and bacteria) so that I would at least have some funky complexity to go along with the sourness, along with some french oak (balanced out the sourness and added some nice vanilla).
This pretty much turned into an experimental beer...
I just went ahead and bottled early with no priming sugar into champagne bottles (wanted to make carboy space for some more intentional batches). I plan to let the bugs do their work in the bottles and hopefully, there will be just enough residual sugar to carbonate (may be low, may be high, happy either way). I plan to wait about a year before opening. My wife and I, though, were quite impressed with what we tasted going into the bottle, so I actually have high hopes for this batch.
With all that said, you're really looking at a balancing game. Herbs and fruits are potent components, so somewhere in the formulation you need to compensate. If you use highly-acidic fruits or yarrow, maybe go for a high mash with lots of unfermentables. If you use strong herbs, go sparingly and choose a fruit that is bold and sharp enough to dominate.
On the other hand, you can just embrace what comes and run with it.
my 2cents
