You have to love the level of clarity behind the EPO Rule Changes.
Take for example R.70a EPC and its application to International applications filed before 1 July 2005 for which no Written Opinion is drawn up:
- New Rule 70a(2) EPC refers to the case “if a supplementary European search report is drawn up”. This implies that a response is required when a supplementary European search report is drawn up. However, the rule then discussing commenting on “the extended European search report” and “where appropriate…the opinion accompanying the European search report”. This implies that an opinion is required before a response under R.70a(2) EPC is required. So far, so vague.
Luckily, the (not-quite-complete, not-quite-draft) Guidelines for Examination (B XII 9) give us some handy guidance:
“There is, however, no requirement for the applicant to respond to the European or supplementary European search report where this was drawn up before 1 April 2010, where it is not accompanied by a search opinion (see XII, 1.1 for applications for which a search opinion is prepared) or where the search opinion was positive (see XII, 3.9).”
So basically the rule seems to be “no opinion, no response required”.
You can, however, file a response if you wish:
“…the applicant may still respond to the search report according to Rule 137(2) if he so wishes. In such cases, the applicant is encouraged to respond to the search report before the application enters the examination stage (see C-VI, 1.1). “
IP Chimp 
There is the question of whether a response to the supplementary Search Report with no Opinion is your only chance to voluntarily amend.
Technically, the new R.137 applies. However, R.137(2) only refers to “…[stuff] made in response to communications by the European Patent Office under Rule 70a, paragraph 1 or 2, or Rule 161, paragraph 1…”. R.70a(1),(2) and R.161(1) refer to “the opinion”, which in the case below is not issued. This suggest you should get a chance to respond “of your own volition” in response to substantive comments (i.e. the following exam report).
There is also the question of (the first) restrictive interpretation being ultra vires given Art. 123(1)EPC.