As a workaround you can register the channel with pear/pyrus then install PHPExcel from a local package.
Stub channel file (pear.pearplex.net.xml)
Stub channel file (pear.pearplex.net.xml)
<channelxmlns="http://pear.php.net/channel-1.0"version="1.0"><name>pear.pearplex.net</name><suggestedalias>pearplex</suggestedalias><summary>pearplex</summary><servers><primary/></servers></channel>
> pyrus channel-discover .\pear.pearplex.net.xml
Pyrus version 2.0.0a4 SHA-1: 72271D92C3AA1FA96DF9606CD538868544609A52
Using PEAR installation found at D:\PEAR
Discovery of channel pear.pearplex.net successful
> pyrus install .\PHPExcel-1.7.6.tgz