I will admit, one of my main fears when switching to a natural diet with my rabbits was providing the enough and the correct nutrition without feeding pellets, however early on in the process my vet offered reassurance that pellets really were surplus to requirements. I read more into the subject and came up with the following guidelines. If you are following the natural diet plan, you will already be providing unlimited access to forage/greens so the doe will be able to perfectly regulate her intake and ensure that she is eating enough throughout her whole pregnancy.
Providing a wider range of collected forage as well as cultivated vegetables during this time will help the doe select the exact nutrients she requires. Ensuring that there is always access to fresh grass available will balance out the diet nicely. You can also increase the dried forage offered during this time as well as the added calcium will benefit a pregnant doe.
You will also find that does will produce far more milk when fed on a natural diet, I personally believe this is down to the fluid intake from fresh greens rather than the dehydrating properties of commercial pellets. Babies will grow strong and hardy with this abundance of nutrient rich milk, giving them an excellent start in life.
With all this considered it seems that feeding pregnant does on a natural diet should offer no issues, however care must still be taken as some plants have medicinal properties that will negatively affect pregnancy:
Lavender - is a strong medicinal herb that has long been used to cause females to violently expel the contents of their uterus, it is often used to trigger a miscarriage. It can be used on rabbits that have miscarried a litter or only partially miscarried. With this information kept in mind, lavender should be avoided completely for expectant mothers.
Shepherd's Purse - similar to lavender but the effects are not as strong, historically it has been used to induce labour. It is therefore reasonable to assume that this plant is also best avoided during pregnancy, although I heard of it being offered to does that have gone beyond their due date.
Mint - can be used to dry up milk, so while it is safe to offer in the early stages of pregnancy it should not be given during the last two weeks of pregnancy or while the mother is feeding her young. However it makes a welcome addition to the does diet post weaning to help keep her comfortable, stem the milk flow and prevent mastitis.
Sage - generally has the same milk production properties as mint with the added property of being a muscle stimulant. It is often reported that sage may stimulate the uterus so again is best to avoid during the whole of pregnancy through to weaning.
Parsley - is a hugely popular herb among rabbit keepers, however it is important to realise that it can cause uterine contractions, which can obviously put a pregnancy at risk. Parsley should not be offered during pregnancy, and shortly afterwards (while the doe's uterus is still recovering from pregnancy) but is safe to feed while the doe is feeding.
Yarrow - I haven't been able to find any exact details or references to the properties of yarrow except that it is often linked to miscarriage so again it would seem best to avoid feeding during pregnancy.
On top of the considerations above there are some plants that can offer huge benefits if fed during pregnancy:
Blackberry & Raspberry Leaves - provide cooling properties (by increasing blood circulation) which will be greatly appreciated by pregnant does in the hot summer months, they are also believed to aid milk production.
Comfrey - is known to have calming affects which will benefit pregnant does but is also very high in vitamin A which is vital during pregnancy.
Nettle, Goats Rue & Milk Thistle - are all very high in calcium which in tern aids milk production, ideal to feed in the latter stages of pregnancy and while does are feeding.
Dandelion - is a very popular plant to feed during pregnancy it is high in calcium and vitamin A.