You are looking for the msg argument, which, by the way, will ignore img and msgText .
loading : { msg : $('PUT YOUR HTML HERE') // Accepts a jQuery object }
Source: jquery.infinitescroll.js. Inside function infscr_create :
// Define loading.msg opts.loading.msg = opts.loading.msg || $('<div id="infscr-loading"><img alt="Loading..." src="' + opts.loading.img + '" /><div>' + opts.loading.msgText + '</div></div>');
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