Can a release be identified by an ISRC?

No.

The ISRC is a unique identification system for sound recordings. Sound Recordings are not Releases. Formally, DDEX defines a Release as:

An abstract entity representing a bundle of one or more Resources compiled by an issuer for the purpose of distribution to individual Consumers, directly or through intermediaries. The Resources in Releases are normally primarily SoundRecordings or music audio-visual recordings. The Release is not itself the item of trade (or Product). Products have more extensive attributes than Releases; one Release may be disseminated in many different Products.

In less structured language, Releases are the collection of Sound Recordings and/or other Resources that can then be productised and made available to consumers under specific terms and conditions (a.k.a. Deals).

So for this reason, ISRC codes are never suitable identifiers for any release types which incorporate several resources, audio, video, images, or text. For example a Single track audio single, contains two resources, only one sound recording but also a cover art image. The same applies to a ringtone, one sound recording but also one cover art image. For all those releases, the main release identifier has to be either a UPC, or a Global Release Identifiers (GRIDs). These can be augmented by a proprietary Identifier, such a catalogue number for example but the UPC or the GRID are the only primary release identifiers permitted.

This rule has two exceptions: Track Releases and Single-Resource Releases.

Track Releases 

They are Releases that contain only one sound recording or video which form parts of the main (parent) Release. 

For example, when communicating a 10-track album, a typical NewReleaseMessage would contain, besides the Main Release, ten Track Releases (i.e. one for each sound recording that together make up the album), so altogether there would be eleven Releases.

The track releases also need to have an unique identifier each. This can be a UPC or a GRID, or alternatively a Proprietary identifier. One such Proprietary Identifier commonly used to identify Track Releases is a concatenation of the main Release's identifier and the ISRC of the Sound Recording that is contained in that Track Release.

For example:

<TrackRelease>
	<ReleaseReference>R1</ReleaseReference>
	<ReleaseId>
		<ProprietaryId Namespace="PADPIDA2013042401U">00094631432057_JPTO09404900</ProprietaryId>
	</ReleaseId>
	<!-- ... -->
<TrackRelease>

Note: the example above is in ERN-4 syntax. In ERN-3 the Track Release needs to be communicated in a Release composite.

Single-Resource Releases

These Releases contain, as the name indicates, only one Resource. For example, one Sound Recording but unlike a Single Release, they have usually no cover art.

Typically, these Single-Resource Releases are used for fingerprinting purposes and not for direct commercialisation to consumers, which is why extra elements like cover art images for example are not seen as necessary.

Again, like Track Releases, they need to be identified uniquely. For these Releases UPCs are not the best fit; but GRIDs or Proprietary Identifiers are used.

As above, Proprietary IDs can be the concatenation of the main Release's identifier and the ISRC of the sound recording that is contained in that single resource release, although in that case, the main Release is not included or even referenced in the NewReleaseMessage – but this approach  guarantees the uniqueness.

Another solution is to use the IRSC but only by making it a ProprietaryID.

This is the only case where it is possible to use the ISRC as long as it clearly set in the context of a ProprietatyID and not directly as a Release ID and also only in the case of the Single-Resource Release: