Although I have thought about solutions for carrying items, and experimented a bit with a few approaches, I do not have the experience needed to comment meaningfully on this matter. However, I did learn one relevant fact that is general (not specific to magic); I was surprised when I learned this fact, and am mentioning it here in case it might be of interest to others.
The fact is that the design of holsters appears to be very advanced; it comes under the subject of "concealed carry [of weapons]". It appears that if one is wearing any cloths at all, there is a solution for carrying a weapon in a concealed manner. Moreover, a shoulder holster does not require a jacket to remain hidden -- shoulder holsters can remain concealed with as little covering as afforded by a vest. One can see a wide range of amazing concealed carry solutions demonstrated in this video: http://www.youtube.com/watch?v=ogGBPVk5GQk
Now for some connections between the above general fact and magic:
As another indication of what little it takes to conceal a shoulder holster, is worth noting that the owner of hcmagic.com says that his motivation for inventing his holster type products for magicians was, not for use with suites but for use with flimsy summer cloths! (I am _not_ saying that his products are unsuitable for wearing with suits -- they are indeed suitable; I am just saying that his motivation for their invention was as a solution for carrying items while dressed in minimal summer clothing.)
Incidentally, on his web-site, he does mention that since the police can carry weapons concealed in shoulder holsters, one ought to be able to believe/infer that one can carry items for magic using his products. Not having paid any attention to the dress of policemen, this argument did not carry much weight for me; also, the policemen I have happened to glance at have appeared to be bulky and so concealing things on them doesn't seem to be a challenge. However, after I saw the video linked to above, I readily grasped how a solution for concealed carry would be applicable as a solution for carrying items for use in magic.
I have come across reviews of products from hcmagic -- all are extremely positive, and people rave about the product they purchased starting from the way it is packaged for shipping.
I have interacted with Joel Broock, the owner of hcmagic.com, regarding pre-sales matters. He has been very patient with my inquiries and has responded promptly, clearly, and in sufficient detail to all my questions. It has been a pleasure learning about his products.
The pictures of his products on his facebook page are much clearer than those on his web-site.
Incidentally, I came across a few negative comments about the use of scantily clad, slim models (sylphs?) in his advertisements. I found such comments surprising; I think the use of scantily clad, slim models in his advertisements is very appropriate because such pictures vividly/eloquently illustrate how well hidden Joel's products would be on the people he is selling to, viz., magicians.