Well, maybe. Attitudes on imm have become more positive but voters strongly associate pro-imm posns with social liberalism. So a pro-imm pro-Remain party would be joining a crowded "Lib-left" field (Lab, LD, Grns, SNP all seen as being in that camp, rightly or wrongly) 1/2
As @p_surridge and others have pointed out the big gap in the market is left-authoritarian views (economically egalitarian but socially conservative). Hard to credibly appeal to that segment without being somewhat restrictive on imm.
(PS I'd rate the chances of this particular initiative getting anywhere as pretty low in any event).
