How can I tell whether transactions were (not) made via BitPay / Coinbase? Background

Recently, Rep. Steve Stockman posed for a photo op with his Bitcoin address on a big QR code, which got shared. He has a Bitcoin donation page which asks for identifying info and goes through BitPay, but of course anyone could send him Bitcoin anonymously, without using that page, if they know the address to send it to.

The Libertarian Party and CoinPAC have Bitcoin donation pages that don't outright list their addresses, but block chain analysis of a donation would probably reveal their main address(es), and those addresses might have received contributions that didn't go through the donation pages. (The LP's page doesn't even require giving your info for it to show you the donation address.)

Stockman &amp; LP's donation pages use BitPay; CoinPAC's uses Bitcoin Builder (which in turn uses Coinbase).

The Livingston County Libertarian Party (addr) and Jim Fulner (addr) publish their Bitcoin addresses openly on their websites, and the Bitcoin Voters PAC published a list of their Bitcoin addresses in their official FEC registration filing‡. None of them even have Bitcoin donation pages that collect info, so of course anything sent to them presumably wasn't properly attributed.



Problem

The FEC requires PACs to know the identity of all contributors (especially for "in-kind" contributions, i.e. anything other than fiat currency). If any of the above received any Bitcoin contributions that didn't get attributed to a specific person (e.g. by going through a donation page that requires disclosing your ID before telling you the address to donate to), that'd probably be an unlawful unidentified in-kind contribution.

For our upcoming FEC advisory opinion request about how to handle Bitcoin, I'd like to have example(s) of how:


it's not possible to "refuse" a Bitcoin transaction (e.g. someone contributing to the above outside of their donation pages) 
it's not possible to reliably refund a Bitcoin to the control of the person who sent it (e.g. a transaction that was laundered or came from an exchange) 
some transactions are worth less than they cost to get rid of (e.g. dust or complex / intentionally large-byte-size transactions)


Our request proposes a framework for how to deal with these problems (e.g. one could donate unattributed Bitcoin contributions to a charity). I just want real-life examples I can point to that show it's not a purely theoretical issue.



Question

How can I show that Stockman, the Libertarian Party, and/or CoinPAC have received BTC contributions that did not come via their donation pages (and thus BitPay or Coinbase)?

Pointing to specific transactions they received and explaining how it is clear to a third party that those transaction(s) weren't made via BitPay/Coinbase would be extremely helpful.

Other examples of political committees that have published a Bitcoin address or are currently receiving Bitcoins in some way would also be much appreciated.

Thanks in advance!



More info

† See our bitcoin AOR page &amp; my G+ post for some more background.

For the record, we are strictly non-partisan. Our intent here is not to make any particular candidate or party look bad, but rather support transparency in campaign finance. 

We want to accept Bitcoin  (potential donors have requested it and we think it's a good idea), but we think it needs to be done carefully to be compatible with current law and good public policy.

Comments on our AOR are welcome, but not really on topic here; I'm just pointing to it for context. The question here is simply: how can I identify a transaction as being / not being made via BitPay or Coinbase, just looking at the block chain? (Let's assume there's no subpoena involved. :-P)

‡ So you needn't transcribe:


18kKJ15uz1nuMJ2kacuahLKLL9NwnsbRQi
1EsR4cdbgPvnD3Gk6wNkTotdzexiN56wXt
14jc8MtHxE6M6Xn7LarvZnXF6GgaaRy1PY
12QeAyDyXiMVn5uwpQUHwS8g2Z3SZf2Kvx
1GuHY98Bxf4mFw2CQEUXqwDUTRqnTRrCt8
113pdko8qQYsvkbV4r7H247oFPFNMnUYqY
1M9AfWnRFmgQgAuTsQssL523ANsxKzfygs
199JdcXMQ41eA9ZPTLUktR9kfAiQRhaErW
1AZxfdTXg5V3gRZMtMfQZrwgBeaQMAtKqN
1NyuGppGprR6uM2koN7hH8wGP8bk6wKh15
Only way is if Coinbase/Bitpay publicly released their addresses, which they won't.