No, there is no reasonable way to estimate those carbs.  If you're really
concerned about it, avoid the practice. People do pick the toppings of pizza
and such. Picking around the carby items in a meal every now and then is one
way to deal with the situation, and I see no real reason NOT to do that.

Again, if this bothers you, avoid the practice. Dieting is more about what
you do over the long term, not what happens at a single meal.  So, if you're
faced with this situation, just do your best.  If you manage to do fine the
greater majority of the time over a sufficient time span for weight loss,
you'll be successful.  You don't have to do the diet "perfectly".

I would find it much more annoying to eat this way than to just cook a
bit in a separate pot for myself.

I might do it if I were eating out from a buffet or something.  But for
the most part, trying to separate stuff that has been mixed is way more
annoying than just not mixing it to begin with.

If I cook a roast with onions and garlic, I eat my portion with onions
and garlic, so I count the carbs.

I don't think there's any accurate way to estimate how much carb got
"into" the meat versus how much is left in the sauce, so I don't try to
do that.  I just make the entire recipe "legal" for my way of eating.

If I need to make it higher-carb for someone else, their food is put on
top of pasta or rice or potatoes, or served with some carby food on the
side.

You might be interested in my site, cause that's actually *exactly* what
it is about, how I manage to eat low-carb while cooking higher-carb for
my husband, without having to cook two entirely separate meals:
http://ornery-geeks.org/text/cooking/

At restaurants I find that it depends on the size of the meat chunks.  With
large meat chunks the meat can be cleaned to the point that I might get 10
grains of rice for 5 chunks of meat.  IMHO that is insignificant carb
poisoning.

If the meat bits are tiny, forget it.

Cubit
190/157.3/160  (originally 320 in 2003)

At home it is far easier to cook real food for yourself as one
adaptable dish.  Then cook some starch dish on its own for
the skinny folks.  Then serve then both but you only the real
food.  That way picking the real parts out isn't an issue, adding
the starch in is the issue.

Use actual garlic and it isn't as much an issue.  Then as your
carb quotas evolves you'll have room to use the mix.

I use the onion soup mix in sour cream to make a dip to put
on my celery.  Since I'm sure I eat all of it, I count all of the
carbs listed on the label.