Based on a paper "Pharmacokinetics and brain uptake of an IgG-TNF decoy receptor fusion protein following intravenous, intraperitoneal, and subcutaneous administration in mice." by
Sumbria RK et al. there is a difference and SQ is the mode to prefer in this case.
The crossing of the BBB isn't necessarily contingent upon the route of delivery and has more to do with the phyicochemical properties of the compound. The question of IP vs. SC has more to do with systemic exposure. IP injection may reach a higher Cmax (more quickly), and if the substance you're interested in is a transporter substrate, there may be a chance of saturating uptake and/or efflux from the BBB compared to the slower absorption typically observed after SC dosing.